The biggest reasons people quit.
Toxic players that troll other ways No feasible way to prove this. Off-meta picks. Players not caring about matches.
Main champions getting gutted Partially due to the balance team Too many champions Too many bans
Matchmaking is awful solo duo and players naturally troll flex You can't duo with your friends. You can't do anything with a full premade that matters. No incentive to group in a 5v5 "team" / "competitive" game.
But hey, at least players are still buying skins Suffering from trolls, bad matchmaking and we still rough it out just because we've played this game for so long. There's literally no incentive for senior players to stay. There's no incentive for older players to come back.
Riots stance on reworks and changes effecting pro-play What about every single tank pro player? Support? Even mid laners are feeling it.
We're so restricted in a game with so many choices.