Yes it's popular, I play it more than actual LoL to spare myself the rage inducing games of Ranked.
I mean it's not that old brah, you can't expect everything and you can't just port games from one programming language to another when they are as complex as a game like TFT because of syntax in the programming languages. A TFT client would require a whole Mobile client, which takes time other wise they'd have to redo the PC Client as well instead of having incorporated into the LoL client.
Mobile in the end they can have match with PC players, but with TFT on PC being a part of the actual LoL client/game it's not mobile compatible thus in needs in programmed into a langauge that is. A game that complex and popular needs some hard work and actual testing before it close to a public release.
I'm just waiting for LoR; that I really wanna play as an old Hearthstone player.