I wonder why TT died
You can’t finish missions in TT or rank up your champion mastery at nearly the normal rate on TT so that already makes the average player never play it by itself.
The last patch for TT was in 2015 and TT would have been better off if that patch never happened because the tower changes badly hurt the game mode.
Even tiny indie multiplayer games that wish they’d have a playerbase even close to as big as Twisted Treeline put far more effort into their games continuously than Riot ever put into TT.