It's a simple business model:
More options for a set amount of players = less players for any single gamemode
Under this fact, we know queue times will increase as the lack of compatible players to be matched with decreases
Another fact:
Interest in special gamemodes do decrease the longer they are made available.
Knowing this, as people generally leave gamemodes such as URF after awhile (and we are talking about the community as a whole, not your personal preferences) queue times will increase even further for the select few that continue to play these special gamemodes.
It splits the community and is an overall waste of time/money/server space for both Riot and the Community