For one thing, lack of ranked was a significant factor in Dominion's lack of popularity. Calling the Crystal Scar a "failure of a map" is more than a little harsh considering Riot didn't give it the opportunity to thrive. Forcing a competitive game mode into a casual queue doesn't really give a clear goal there. Even then, the mode was still fun, and people still sought top-tier stats there.
Secondly, Treeline is just as much of a failure as Dominion was, in that neither of them are SR. It's not a secret that Riot generally neglects Alternate Maps. There hasn't been a balance update in months. Mastery for the upcoming rotating mode was somehow deemed higher priority than the existing TT/ARAM queues, and Crafting drops are bugged for non-SR maps. Plus none the ranked TT issues this season haven't been acknowledged by Riot.
Finally, why a ranked solo queue for TT? The mode heavily relies on teamwork and coordination, starting from champ select. Although having more ranked options would be nice, having less coordination on TT doesn't seem like it'd be particularly enjoyable.