Regardless of how big your organization and its fans are, anything which compromises the top level competition is going to be a bad thing. In the event, however unlikely, that dom or TT gets ranked play and full esport support, the last thing Riot wants is to weaken their pro circuit by drawing players away from SR. Ranked or no, that's why you'll never see TT or Dom get Riot's full esport support. They probably won't mind if their pros play around between maps, but ultimately the focus shouldn't stray.
As an example, and this is a bit of a stretch, I know, stay with me: imagine if a new type of basketball came out with a circular court, two extra hoops, and only 3 pointers and dunks counted for points. Imagine the new game gets pretty popular, not as much as regular basketball, but there's quite a few new teams who spring up specifically for that court.
Being mostly the same game, a regular pro basketball team could probably come in and hold their own, if not win outright. But to do so would require them to restructure their way of thinking about bball, just a little. The NBA doesn't want to support the new court, because it wants it's players to be totally honed for regular bball. They don't want to deal with "this team might not be as solid as normal, because they've been playing the other type of bball". Or even worse, "This team is better for playing the other bball, and it shows, and now all the standard teams need practice/leagues at both, or else they'll fall behind". It's much more simple for the NBA just to ignore the new court, stick with the old, and stay focused on what they know.
Thus, SR remains dominant, regardless of how much support Dom or TT gets on the side. Because you aren't drawing from a different a wholly different pool of top end players, Riot doesn't want to risk splitting or distracting their pro circuit in any way, as that might compromise the circuit, and it just might not (big maybe, here) be big enough to sustain multiple maps.