I'll start this out by saying it's not because it's "not played". I know there are many players who enjoy or even exclusively play this gamemode, so this post isn't going to be about that.
So I'll be honest, I don't really care too much about TT. I might spam it this season to get the victorious chroma but that's about it. I don't have a distinct bias toward or against TT, I'm neutral.
The problem is, Riot already announced and confirmed that they are removing TT, and that's that. Riot cannot feasibly go back on this decision, or else the community will go nuts. It's the same reason that Riot can't revert VGU's, even for objectively bad ones that very few people like. Riot cannot set a precedent that they will revert big decisions like this, or else the community thinks that Riot will just revert anything with enough backlash.
The only things that Riot have reverted are mini-reworks such as Leblanc, and that's kinda small so it doesn't tarnish their reputation much. Also they reverted making Nexus Blitz a permanent gamemode which had a ton of promise, but we all know where that went... :(
The only reason I'm making this post is because I think there's essentially a 0% chance that this decision gets reverted, and I don't want a lot of people making posts about it because it's almost like spam. It hasn't happened, this is more of a preventative measure than anything.
It's unfortunate that they're removing a real gamemode with real players to cater to their little TFT baby, but oh well.