It has to do with competitive games' win rates. A competitive game's wins are determined either by difference in skill between players, or chance. More specifically, difference in skill consists of the abilities and game knowledge of the one player minus the abilities and game knowledge of the other. Chance consists of game knowledge that neither player possesses because they are insufficiently skilled, or of game elements that are inherently probabilistic and insert some variance automatically. As long as the difference in skill is non-zero, wins will favor the better player; but the wins will be a lot more consistent if the skill difference is much bigger, or if the chance element is much smaller.
The chance element is much smaller in a well-studied game, because there is less game knowledge that neither player has. And Wings of Liberty has been well-studied. At this point there aren't a lot of nooks and crannies in strategy that haven't at least been given a cursory examination. Oh, there's still all the room in the world to improve, and players will do so; but one seldom sees a strategy on ladder that they haven't seen at least once. This fact is exactly what makes exhaustive studies of the game like the recently announced comprehensive PvZ guide possible.
For a while HotS was similarly predictable, because the changes were relatively few and most people followed their WoL strategies with the occasional HotS unit tossed in. But patches have radically changed this game, and to add even more complexity, the game keeps changing in beta patches. Certainly people come up with little tips and tricks to survive, but often their understandings just come out wrong. It's a world where instead of building 5 barracks with tech labs as your push moves out because you know that zergs usually get ultralisks out around 15 minutes (making that up off the top of my head, but it sounds about right), you see your opponent go ultralisks and you stop to think, ultralisks? I guess I make marauders? Or are those even good against ultralisks any more? I guess I could make tanks. Or thors. Or maybe banshees?
I say all this for a few reasons. First of all
Sorry I didn't have any pictures to break up the text. Screenshots of HotS and WoL gameplay probably would have enhanced readability, but I'm lazy. I'd like you to imagine the most mind-bogglingly awesome action shots placed between paragraphs above. Or you can imagine naked women up there, or whatever else you prefer.