Is there a reason behind long dps ques?

[Lumivyory]

Not just MMOs. It's literally every game that has roles. MOBAs aren't much different in that regard - the players favor the roles that bring the hurt and do the killing, not the supportive ones. Unless the supportive ones are better at killing than the designated killers, I've seen that happen once or twice as well >_>
But that's really the only exception I ever witnessed. Otherwise, killing roles pretty much > everything else as far as popularity goes.

Same with shooter games like Overwatch, for example
TFW you get into a competitive match and people pick 4 dps and refuse to switch... Sigh

This isn't complete correct. This includes all all unlocked classes and jobs, even if they are not played.

I suppose but far not everyone has all their jobs at level 70, just their mains and a couple of others so the level 70 one is more or less correct