This. Also, a lot of players have ARAM only accounts, really taking away the "random" aspect of it, especially because Ranked removes f2p champions. All ARAM would consist of is players who buy the most broken things on it, removing the fun of the mode.
I'd be fine with an ARAM ranked to some degree if it enabled ALL the champions for EVERYONE, making it REALLY random and not just favoring players who are trying to win through RNG. Team compositions are really only overbearing if a team has too much of one aspect, it usually being them having excessive poke/harass and you having no safe waveclear. Most ARAM only accounts were only poke champions, so opening it up to EVERYTHING removes that advantage.
The only other problem with an ARAM ranked is the queue time. With so many game modes an ARAM ranked queue can possibly take forever. I already have 5-10 minutes queues and I feel a ranked would be even worse then that, basically meaning you'd spend as much time in queue or even more then actually playing the game.