ARAM is like the sugar rush, Rift is a cup of tea. (it's not everyone's)
There's a level of patience involved with watching health, minding cooldowns and generally being aware of more than just one lane. Objective timers, the general presence regarding Herald/Baron, keeping on top of (denying) Dragons, all mucho importante-- but lemme tell you, I have a lot of ARAM under my belt and they're just as bad there.
It's a total lack of game sense. There's a lot of bashing, "gotta get that last kill" mentality I see where people are more apt to dive and die for a kill than to try and force their presence out of lane. And that's talking ARAM, where kills should have some level of benefit comparatively.
Honestly, it's not as if they magically turn into good players. I'd argue a lot of them are worse because dealing with potential five-man-focus all game seems to escape a lot of them.