I would guess that a ballista would provide more launching capacity per unit of weight of the launcher than a trebuchet and would be shorter, easier to hide.

Launch frequency may also be a factor – like, you might want to launch multiple UAVs off one launcher quickly for a massed strike.

I remember being a little surprised that the Switchblades used a compressed air launcher instead of those elastic band launcher things, but it might make sense if the goal is to be able to keep quickly lobbing them into the air quickly or if having a compact launcher with rapid teardown/setup is critical.