As I understand it, being a pro e-sports player doesn't particularly come with the type of salary being a professional player for a non e-sport does. For example if TSM were to bench Dyrus, I don't think he'd be making six to seven figures or more a year just streaming at the gaming house.
I think there's also the fact that, well, no one's getting any younger. I remember there was an article a few months ago that I saw that claimed that at least for Starcraft, the pros abilities' basically peak at 21. The number seemed odd to me, but I can imagine that the kind of ridiculous reflexes you'd need to play at a competitive level may just naturally dull over time.
Also there may be the factor that at least here in NA, E-Sports aren't really a thing like they are in Asia. Tell someone here in America that you play video games for a living and they'll most likely look at you really weird or otherwise not believe you. The Asian players, it seems, didn't always get a lot of support or understanding at first, but now their families appear to support them completely. I wouldn't be surprised if there were some families of NA players who, even though their child/family member has gone pro, are probably still disbelieving or flippant about it.
I'd be willing to bet at least some of them hear cracks about getting a real job or going to school to get a real job. Over time, with the stresses of playing SO MUCH and for the stakes you're playing for, I can see where maybe a player might just cave in and go "I've had enough. I can't keep doing this." for whatever set of reasons. I speak from experience when I say that getting paid to do something can sometimes not be enough, even if it was in the beginning. Job-satisfaction IS a thing. If they feel the stresses aren't worth the rewards or they've simply decided to move on to something else and keep League as a hobby, it's definitely understandable.
Sometimes you just want something different in life or from life. Sometimes you feel your age and you realize you're starting to be unable to keep up with things like you used to. Sometimes you're just...ready to move on.
