The real reason we get ARURF instead of URF
Riot has said multiple times (and the players here on the boards have reiterated it) that URF suffered from a stale meta. The same champions every game.
Right, we saw the same meta OP picks in URF every game. The following is a non-comprehensive list of champions I have seen various players complain they have "seen every game" in URF:

So the real reason we get ARURF instead of URF? Count them, somehow in a 5v5 game mode we see 98 champions every game.
Like seriously people, get a life. That's 98 champions that are perfectly strong in URF and viable picks in to anything. The excluded ones aren't even excluded because they're bad, but simply because I haven't seen anyone complain about them being OP.
URF NEVER had a problem with too few champions being played. This is roughly DOUBLE the champion pool we see across 16 different teams throughout the world champions (83 games played). I'm sorry if the champion you like to play doesn't happen to be as OP busted in this game mode as the others are - but that's no reason to make a mode that many others had fun in (by getting to play what they wanted to play, rather than needing to spam 50 games to get the one champion they want to try in the mode) generally not super-fun but only kind of fun.