I'll bang out some paragraphs for you...this will be an interesting exercise in memory, and I'll undoubtedly get some facts wrong:
Starcraft was my first competitive RTS. There was a whole brood of very strong RTS players who had played Warcraft 2 on Kali [an old IPX online gaming platform] who were able to translate many of the skills and principles they learned from that game and dominate competitive Starcraft during the first few years. I came from something of the 'new school' and played almost exclusively on battle.net at first, but started frequenting the Kali community just prior to playing in the 3rd season of the PGL [Professional Gamers League, now defunct.] I managed to come in 4th [Grrr won, Soso came in 2nd, {PH}Boo! came in 3rd, Kain-the-Feared came in 5th] overall, and pocket $2,500 and a new computer in my first legitimate tournament. Playing on Kali gave me access to a much higher caliber of player [at the time it was the most skilled SC community in the world] and I was able to improve dramatically as a player in the months following the PGL.
Despite a fairly non-existent infrastructure of tournaments / competitions the hardcore community continued to thrive...because, well, the game was amazing, and new strategies / tactics / nuances were being uncovered on a daily basis. Back then the official Blizzard ladder was actually functioning and relatively well policed by Blizzard officials. In addition, the Kali community had a ladder of it's own (http://www.casesladder.com/). I spent most of my time competing on these ladders, playing against the best players I could find, learning 2v2, and just generally attempting to enjoy the game and improve myself as much as possible. The PGL and another online league, i2e2, were the only real sources of profit one could derive from the game at this point, but both of those leagues floundered since there just wasn't enough general interest in e-sports to support them. The PGL had plans to host a 2v2 tournament which never materialized [much to the chagrin of players who had put in hundreds of hours training] and eventually they were bought out by http://www.firingsquad.com/ [where Thresh, an old school Quake god worked.] i2e2 did manage to pull off a successful 2v2 tournament, though my partner {PH}Pepe! and I failed to advance to the final rounds. I had considered us to be one of the top 4 teams in the world at that point, so this result was disappointing.
Eventually the official Blizzard ladder began to decay. Win-trading and hacking were becoming commonplace, and playing on battle.net against anyone but 'known players' was a risk [though even against 'known' players one could never be sure.] A popular hardcore SC community site "Tao of Gaming" eventually hosted it's own ladder, mostly for the Kali community. Later, another ladder [Cloud Ladder, I think?] took the reigns when the Tao of Gaming ladder fell apart.
I should also note in here that Koreans started popping up on battle.net in greater and greater frequency. Both Starcraft and SC:BW were released later in Korea, and it took them awhile to get up to speed. What one noticed about them at first was the relatively mechanical and robotic style of play. The way many Koreans seemed to learn to play was to master a single racial match-up on a single map and come as close as possible to perfecting it. They followed build orders rigorously, and often very efficiently, but didn't adapt well to new situations or creative responses by their opponents. Often they would play a moderately strong game for the first ten minutes of the match, but once the game became more dynamic and opened up a bit they would have difficulty doing anything other than throwing the same mixture of units at you over and over again. Obviously, this didn't remain the case for long.
[I should note here that the above is an attempt to paint a generalized picture of the the 'standard' Korean play-style and approach to the game during these early years. I'm just stating the overall trends which I saw in Korean play; there were certainly exceptions.]
Personally, I had started college, while still playing Starcraft off and on. Mid-way through my first semester I was approached by a well-known Kali community figure named General~Khalsa and offered a business proposition. His plan [along with a former webmaster of Starcraft.org named 'Hone'] was to create a pro gaming team [at the time basically unheard of in the RTS world] composed of foreign players which would move to South Korea and compete in the burgeoning Korean professional Starcraft leagues. This seemed like a pretty insane / fun opportunity, and I eventually decided to go. X'Ds~Grrrr and Maynard [and later Thor] joined as well, and the four of us showed up in Korea in early 2000. We stayed in the international dorm section of a prominent women's college in Seoul for the few first months [Ewha Women's University] and made slki bang [the home of the best Korean pro team at the time] our training space.
During the 3-4 months I spent out there, the group's results were pretty lackluster. I had managed to pick up the 4th seed out of 128 players in a KBK tournament, the first of out tournaments during our time over there, but only made it as far as the round of 32 in the tournament itself, mismanaging some reavers against a sub-par Korean zerg player. If I recall correctly, Grrrr made it to the round of 16 before losing as well. Another tournament highlight of mine including losing due to *score* when a tournament began running late and the tournament director decided to stop all of that round's games and declare whoever had the highest post-game score to be the winner. This is, of course, ridiculous, as the post-game scoring screen was never intended to be utilized in such a manner, and in the case of my game gave the victory to my opponent when I had a clear edge. We played in a 'foreign vs Korean tournament, one of our few successes during our time there [with Grrrr winning and Thor coming in second.] We lost a 3v3 [or 4v4?] tournament in the 2nd round. And I qualified for tournament called "Tooniverse" but only managed a 1-2 record, not enough to advance to the next stage of the format [this is the only tournament of mine that actually shows up on the Team Liquid Progaming Database.]
I had personally expected much more of us as a group. To be fair, though, the Koreans were just more dedicated than we were, and the formats of the tournaments made it nearly impossible for anyone but the best to succeed. After a few months it was clear to me that I just wasn't enjoying the game like I used to, and that I wouldn't be able to compete against the best in the world without this passion. I was also very homesick, and wanted to get back to college. Pro gaming, while a nice dream, didn't seem like it would lead anywhere useful. Maynard and Thor stayed for a bit longer, but both eventually returned back home [to Texas and Canada respectively.] Grrr continued to live in Korea [and still lives there today] and also managed to have some good competitive success over the years that followed.
I still played a bit of Starcraft after returning to the States, and actually won the USA WCG in 2000, beating Tsunami in an earlier round, and Jolly in the finals. I decided not to take the trip to Korea, though, as I knew my chances of winning were slim and I wanted to focus on school. A year or so later I came out of semi-retirement after being invited to 'LAN5ARENA' in France and played there as well, though lost to a fellow named 'zarma' in frustrating fashion in the 3rd round of competition. This was the last official tournament of my career.
To answer your exact question, I quit SC because it stopped being enjoyable. I played the game for thousands of hours over a 4-5 year stretch, and I'd had enough. In the past 5-6 years I've played maybe seven or eight other RTS games, becoming one of the top players in each of the games I dedicated more than a month to [this is much less of an accomplishment, though, given the relatively small playing pool of many other titles in the genre.] Starcraft remains the best computer game I've ever played, and probably the best computer game I ever *will* play. So much of my life is intertwined with it, and it's had a huge impact on the direction my life has taken [from working as a designer on SC2 to playing poker for a living today.]
So, yeah, that was sorta rough, and I'm CERTAIN some of those facts / timelines are wrong. But it's reasonably close. Let me know if you want to talk about any aspect of it in further detail.
And sorry for creating a wall of text for those of you who don't actually care.
:7