As far as i know they stated like all other of these new game modes they obviously dont want to favor one over the others. So if one stayed it would have been wise to keep all the others right? if that was to happen then that AI-bots/customs/Normal/ARAM/RANK/TT/All-4-one/Showdown/URF//Doom-Bots/Accession, plus anything new they come up with. the queue time would be horrendous!
Not only that but despite it being a "joke" mode a lot of people took it serious and because of that a lot of champions were getting banned. i would think if it stayed out for this long there would probably be a handful of champions to pick on. Along with that because people were taking this game mode a bit too serious a lot of toxic people were coming out which i doubt Riot, let alone the community wanted. I remember getting yelled at because i wasnt playing X-champion because they were "OP". i couldnt have fun with the champions i wanted to play because people only had winning on their minds for this joke mode.
With that i believe someone on these forums said it best. Riot obviously didnt want to change things just for this match. i mean it did happen. look at the rework Soraka got. i doubt any of that would have happen if there was an URF mode. people would spam her Q and because of that it got nerfed till it was completely changed. now think of all the other champions that could have been hit?
Im not bashing URF mode i just feel it would lose its enjoyment if it was kept as a permanent mode. it was something fresh and out of this world. if it was to stay it would have gotten dull and boring. i know i was getting bored of it when they were banning half the champions. it would only be a matter of time before others would have as well till it would have been a dead mode!