Not Available

Guess the Sulaco was available.  The Nostromo was gigantic for seven crew.  300m long spread out over 3 decks.  The refinery where the ore was was about 6 times larger than the towing vehicle.

I think i guess they felt for the size that was Hadleys Hope Colony, and what little they was expecting....   they may have felt that sending a full complement of Marines was a Overkill and not needed.

@BigDave

Ooohhh! That makes a lot of sense! They probably did have a mass evacuation in mind when taking the Sulaco. Thank you for the answer, clears up my mind ;)