Actually, I remembered why they couldn't send back one of those big-ass harvesters. I thought for a minute "Yeah, why DIDN'T they do that?" and the answer, story-wise, came to me. I won't say what it is, because I don't want to be THAT guy. Just think back to the first movie, and some of the rules that were established there.
Although I think the reason the guys pointed out is probably the more practical one, and the rules of the franchise were written AROUND that, and it still cracked me up when they said it.