League of Legends is a competetive video game which means some people take it seriously. It's not "just a game, who cares, move on" for them.
But, for some players it is. They just want to have some fun: "who cares about winning, I wanna try Alistar mid!". Or when they get autofilled: "whatever, I'll go ham with my god tier Yasuo wish me luck XD".
Also the game is getting quite old. Experienced players are getting old, too. They don't have that much time or will to play anymore which, of course, leads to less try hard mindset. Yet they are good at the game but because of lowered hours they put on the game and rank being indifferent for them they aren't playing at the mmr they are supposed to play. Diamond tier player's rank could easily be gold 2.
Caring about team composition is one thing too. People who don't care about winning just play what they want no matter what their team picks. Tryharders are more willing to play less fun champion to make their team work better together. They get their satisfaction from playing well instead of making cool moves which do not pay off.
Smurfs are a huge problem. They make teams really unbalanced. I tend to play at night and I constantly encounter smurfs in my platinium elo games. They might be former high elo players too, but for example after last game last night I asked enemy jungler who played really well that if he is a smurf or not he said he is duoing with a friend with his secondary aco%%%% because there is no other options to play ranked with him.
I think CS:GO has some kind of tryhard ladder which you can get into by paying a small fee. Why don't we have something like that? A fee would weed out(?) a big part of these players I mentioned above. It just doesn't make sense that everyone plays the same matches. Has Riot ever said anything about this topic?
Toxicity is huge thing for Riot and all this is strongly connected to it. It's really really frustrating to tryhard while others instead of tryharding just have some fun next to you or simply give up after dying twice on lane. It's not fun to stomp or get stomped.