They do not want to make it permanent because it will ruin the purpose of the game, when urf came out this year the first day was the best, everyone trying different things (AP fiora op) and having fun. stalling games out so everyone can have full build and mess around with their champs, etc.
But second day you had all lux, ezreal, zed, hecarim, sona, kindred, etc.
because people had figured out how good they were, they started making a meta
and meta kills urf. kills the fun, kills the manatee, kills the joy. because people get so salty when going against op team comps, and not being able to do anything, because if a champ is kind of op in normals they get even worse in urf, and some get downright scary and can carry a team alone (kat/zed, im looking at you)
but if it was permanent then people would have time to try every type of thing they can do to find the very best ones and every game would look like the LCS (Same champs different game)
I'm glad it's on the rotating que now so it's no longer once a year type deal like it was. you'll see it again bud, let the manatee live in your heart URF!