In contact harvest they had to use Eva suits winch meant no artificial gravity or life support. The UNSC would have to refit majority of their merchant fleet to accommodate evacuations and such.sorry I'm at work, I would elaborate but I feel this point is a pretty good one.
Ah yeah, if they where all without life support and gravity that would make things very different. Maybe the refit of a few hundred vessels could be done but it's still a lot to do considering the covenant showing up was a surprise so unless they planned for an invasion and already refitted the ships that is ALOT of work to do.