Dominion matches actually used to show up there as well. It was understandably frustrating though because there were no labels, and the only way to tell whether it was an SR or Dom match was to check the background of the Featured Spectate box. (Dom had a different arrangement of houses or something, I forget.) Anyway, I don't know if they stopped because Riot changed the algorithm or if there weren't enough high-MMR matches anymore. Unfortunately, as far as I'm aware, Treeline matches have never shown up for whatever reason.
It really would be nice to easily see high-level gameplay for CS/TT. Particularly after the ill-considered changes to match history visibility, it's much harder nowadays to research builds for alternate maps. Having CS/TT show up in the Featured Spectate would help expose the game modes to new players too.