Because Diamond LP gains are completely fucked, so almost every Diamond player I know just smurfs instead.
My friend was gaining 26 lp/win before he hit diamond. Hits diamond, gets 15lp/win. Guess what? He used to exclusively play on his main, but now he exclusively plays on smurfs. This is true for a lot of diamond players I know. The LP gains are so atrocious so they will play on their smurfs instead.
Smurfing isn't just bad because of skill difference, its mostly bad in because these diamond smurfs do not give a fuck and will troll/afk cause its not even their main account. Plus they don't even really want to hit diamond again (shit lp gains).
Also, b) runes being free actually makes it a buttload easier to smurf. People can buy smurfs for super cheap cause bot accounts in pretty much 3 bots vs 3 bots matches (aka no reports). Before you'd need to either spend some time or money to be able to "smurf on people" because you'd need a wide range of runes to be effective. Now you get all runes for free, so smurfing is dumb easy.
You could say runes being free is a net positive, so w/e about this one. Just keep it in mind.
c) Now it's just common knowledge that griefing in game doesn't get punished. Because it's so well known, people have no issue just ragequitting/inting games. Even if the system sucked before, I feel like most people _didn't know_ it sucked that bad. Now it's been popularized by streamers ("riot doesn't punish trolls"), so now everyone knows and does it.
That's all. Not saying they can fix all of this, but fix diamond lp gains. please.