Toxic players are in every mode though. So if that's the case, wouldn't that mean people wouldn't even play the game?
Some modes are significantly more toxic than others. Coop vs AI, ARAM, and (mostly) RGM modes are usually pretty chill. When you get a game on Summoner's Rift, especially if it's ranked, then the mute button gets a workout.
For real though, why does no one play it???
There's no pro scene for it. It has a ranked mode, but when's the last time you watched a pro stream and they had a 3v3? Almost never.
Also, champions are balanced for the Rift and not TT. Riot tends to ignore TT, so players tend to ignore TT, so Riot tends to ignore TT, so players tend to ignore TT, and you see the cycle that killed Dominion forming all over again.
Finally, once Dominion fell all the bots that plagued that mode moved over to TT. I remember my first TT game of season 6 had two human players; me and some guy on the other team. We joked around with each other as we both pushed our respective lanes all the way to the nexus with little challenge. And that's what players get as the first impression: A bot-filled joke.
However, there's still good things about the mode. Nobody has to be a support, and that's major brownie points in the light of this whole autofill debacle.