Why are game modes removed?
Riot in my opinion is fearful of what will happen to the new game modes after they are left out for a prolonged amount of time, because how dominion was received by the community. So in result they make their new game modes available for two weeks and that's it. They disappear into the vaults the community unknown if they will ever return. Many a time players who enjoyed these game modes cried in outrage wanting to play them again.
So why not bring them back in a rotation? Like how Champions are on a weekly free rotation, why can't the maps be in said different rotation? I may be talking out of my ass here but bringing these game modes back once in a while won't hurt summoners rift.