Riot has openly announced that the only reason they don't implement URF often,is due to the fact that it causes them to lose users. Most people are going on a frenzy with URF for a couple of days,and then they burnout and leave League for a while,sometimes forever.
The fact that it still comes back from time to time under a different form is only due to the players' constant request.
Company marketing wise,it will never be a permanent game mode.