They don't keep featured game modes as permanent modes because they aren't meant to be. They are meant to last a short amount of time.
http://na.leagueoflegends.com/en/news/game-updates/features/featured-game-modes-2014
Why are Featured Gameplay Modes temporary?
Featured Game Modes are designed from the ground up to be short-term engagement experiences. This transience is what gives us the creative space try new things and see what works, listen to your feedback, then improve a mode before re-releasing it. Trying to build a long-term sustainable game mode would actually constrain us from doing things like Doom Bots, URF, Legend of the Poro King, Ascension, etc.
The main design concern (which is personally the most important as it = less fun) is that Featured Gameplay Modes are not designed to be long term engagement experiences. They’re designed to make a big splash, with the knowledge they will be removed shortly thereafter. Adding permanency would constrain the design wiggle room we have to make each mode as unique as possible. As we’ve since learned, it would also add additional balance and maintenance costs that we’ll share some examples of later in this retro. For now though, let’s get into the modes!
That says it all. URF itself is a very unbalanced mode overall and would need constant work to keep it up.