URF the second featured game mode after Hexakill.
Hexakill did not get much fans because it was boring, slow and too similar to classic games.
URF that was meant as april's fool joke got so much hype that even though it was meant to stay only for one day, riot decided to let it stay for a while.
As it was only second mode and the first one good, everyone was enjoying it.
After a while it got into stage of tryhards that thought that this mode can be played seriously. (this stage came almsot instantly this time as mostly everyone knew how to play this mode already)
And with the age of tryhards, the URF was closed.
After that people were calling fro URF to come back, but what kind of people wanted that?
Ofcourse the tryhards. But not only them, a lot of other players wanted to taste that sweet taste of speed again. And so bad was forgotten and community agreed tha they want to play it once more.
But more modes came and we had fun with hexakill in TTL, Ascension, One for All etc.
Yet still URF hype was there, and the only reason that it was there was nostalgia. Because it was first mode that was fun.
In other words URF hype is like remembering "the good old times", though you have already forgetten all the bad about it.
But You are not hyped because there were more featured modes afterwards that were also fun.