compared to MoP, Cata was waaaaaaaaaaaay more active (atleast in its peak). It probably isn't fair to compare Cata's "glory days" to the few months MoP has been here, but hell, Cata was way more active.
Of-course, the removal of skirmishes have been a factor of why the twinking population has decreased -- though I can't agree with you that it should be one of the two reasons why queues are slow/does not pop. My conlusion is that "birds of a feather flock together"; many small factors (gy-farming, skirmishes gone, no mobility, OP heals, OP burst, endless trashtalking on TI, winlogging, etc, etc) is the reason behind it.