Also people have figured out how to play against sona in lane. A few months ago I was able to just completely rampage in lane with sona but alot of support players are picking extremely bursty stuff like blitzcrank, new taric and leona. You still can win lane against them as I don't think those champions can prevent sona from doing her job in lane, but ad carries and supports started to play extremely aggressive against sona, as she doesnt have tools to keep champions in place: she just deals damage and heals a little. With lux you can do what I'd like to call free damage: with E range you can safely deny farm if you time it on lasthits or you just do damage without providing a hole for them to trade back. With Q you have a major disengagement and killing tool on a huge range, which punishes mistakes really hard. After 6 you can just R anyone you hit with Q as you are not the ap carry you are allowed to do that on a full HP target which makes much more use of her kit than ap lux btw. If you get ahead with lux support in lane you and you can push the lane then the real fun happens. With sona and janna to a lesser extend you can poke ppl under turret to keep an advantage but any of those champions look pale compared to lux when doing that.
And all that is not even her main strength. I think what really makes her a strong support pick is her ability to roam and ward like no other. The champions I like to roam the most with are blitz, leona and janna just because they are fast have strong initiation tools and have the ability to not die while warding/roaming. Lux is even more of that because of the massive range of her spells and the fact that she can check brushes/etc with her skills is amazing. This is one of the major reasons why venge was so popular when I played dota: you could safely check areas you want to go in while warding/ganking/roaming.
What lux does not have is buffs, so she is in the blitz, leona department of buffless supports. I really love those champions but you cannot pick those in any lane/team comp. I think if you want a poke lane, strong roaming/warding and disables your choice needs to be lux/janna atm. If you are worried about disengaging more than about having strong ganks and midgame power then stick with janna, else pick lux.