They've explained over and over again why these modes aren't permanent. If they keep them around forever, they have to keep supporting them. Every time a new patch or champion comes out, they have to make sure it's compatible with all modes. Riot doesn't have infinite resources--they would rather focus their efforts on improving the core gameplay than supporting one-off modes that would get incredibly boring anyways.
URF was a very fun mode. It was fun because it was wacky, people didn't take it seriously, and it was hilarious to see how ridiculous your favorite champion was. The gimmick would get very old very fast if it were kept around. The mode (which was never designed with balance in mind) would have quickly developed its own meta, which would have sucked out whatever fun was left in it.