There's an interesting storyline being created about that by fans: several members of the platoon were going to go on R&R, and others about to finish their tour, like Hudson, which is why they kept complaining about the mission and were thinking that it was going to be easy to accomplish. This might also explain why Ferro was a Corporal (one wiki says she was originally supposed to be a Sarge, which meant that she would have outranked Apone) and why the original commander was replaced by Gorman (who also worked with Burke).
Both the military and the company needed a unit that could kill "bugs" (hence, the bug stomper logo), and in this case around two aliens reported by the colony (they didn't know about the hive), and right away with the least people knowing, which is why they used the Sulaco, which was about to be decommissioned, but whose captain and crew had already been transferred to a new ship.