My guess is they want to keep the Scum faction as the smallest faction, both in campaign and in skirmish.
Campaign: the core theme seems to be Rebels vs Empire, and the scum are just mercs hired by the empire.
Skirmish: With Rebels and Imperials able to use Scum, they want to keep it from not overlapping with already made cards, if they make too many mercs. (I.E. No mercs that are basically probe droids, no mercs that are basically rebel saboteur, etc.)