Well, I don't speak for everyone but from my experience, I quit because I was in this unhealthy mindset of "I must win!" instead of "I just want to have fun." The feeling of first picking up Ahri, getting my ass handed to me by the many different champions, trying my hardest to learn the matchups and perfecting my skills with the Fox-y lady were the reason I still kept playing up until I plateaued with her.
Eventually, though, every game became "standardized." If you did everything right, chances were good you can get a win in a 1v1 situation and if the enemy team had a better teamfighting comp, you're going to lose almost 100% of the time. I used to live for the 1v1 in the mid lane up until it became habit that jungler camps my lane or I get four-man bot while playing support. I get it's a team game, it's just fights don't really feel special to me anymore.
Back to the "winning and losing" thing, I like to win, but I also loved when I lose because it meant there was something I could improve. However, because there's so many other people fighting, that question of "how can I personally improve" becomes muddled because you have four other people to think about. Warding, vision control, keeping an eye on everyone's hp bars, it's so much for one person to keep track of instead of "hey, if I charm between dashes, I can get this person more easily in case he still has flash up."
If you don't play the Meta, you're going to lose or your team will flame you hard. While it's not pro play, you still only see the same ~15 or so champions in game. For me, the game was stale no matter how much I tried to mix it up.