I'm gonna disagree; Her magic can be used by anyone who knows what they're doing. Sure, she's very good at it; maybe she is a spirit-binding virtuoso, but that's just a skill, It's if someone was raised to play an instrument since birth, they're going to be way better than anyone else, and to outside views, they might appear special, but it's nothing the average person couldn't do with a lot of study. The KR have an outside force, essentially a magical symbiote, that grants them power; they are special because a spren was destined to bond to them. (I'm pretty sure Syl said somewhere that she was always with Kaladin, even when she wasn't, like how she remembers stuff Kal did as a kid?) The only thing that might throw a wrench in my ideas is that the spirits talked to her, but maybe they're just doing that because she is very good at her job. As for why Painter was chosen? Maybe because they were both unsatisfied, causing some cognitive resonance that led to a natural connection the spirits could take advantage of?