I'd say because of mercenary status, that is, since mercenary are available to most faction, you have to take special care in balancing their cost and therefore usually ends up on the expensive side of the hard to clearly trace line of balance. Or as IJW said, you sacrifice SWC to take that cheap minelayer in specific factions, and the other factions are stuck paying that extra SWC sacrifice too.
Merc tax to lower spam on an otherwise excessively cost efficient profile to stop it from being autoinclude. It'd be worse than that it's got FD2.
