Thoughts about ARAM vs. new game modes

While Summoner's Rift is the main reason why I play LoL, I do enjoy other maps and game modes from time to time. I have 742 ARAM wins and have played Howling Abyss quite a bit, however at this point I can't play it anymore because it's too imbalanced and not fun. ARAM is fun when you have two teams that actually feel random, but once you've been playing the mode for long enough you'll end up playing only with people who have "figured out" how to win, which is basically to own all the OP long range poke champs and healers and to dodge the queue every time you have a bad team comp. In contrast, I had a lot of fun playing One For All: Mirror Mode on Howling Abyss, and as far as I can tell it's perfectly balanced (go figure). I've also been enjoying Hexakill on Twisted Treeline so far and it seems to have similarities to ARAM concerning the teamfighting dynamic but I think it also allows for a lot more champions to be viable. There may be some balance concerns here too but in my opinion it seems generally much more balanced than ARAM and thus more interesting. What I'm wondering is why ARAM is a mainstay yet game modes like these don't last longer than a week. They are in my humble opinion much more fun than ARAM and would make me want to play the game more often if they remained. P.S. I also liked URF for a laugh.