One reason is the fact that playing any draft mode will complete all quests automatically, other than the “Win 3 games” quest**.
The one change I would love to see is to allow us to complete quests in ARAM, since they take basically as much time as a “normal” match anyway, and it is far more likely I will end up playing an assassin (or an Overwatch hero) in ARAM than in QM.
** Which I swear is a curse. As soon as I get that quest, I feel like a 7 game losing streak is incoming.