And why have this giant ship that only carries one platoon? Why only send one platoon in the first place?
A lot of what I'm about to say is nothing more than supposition but here goes...
All (the "suits" at Weyland Yutani) knew was that the colony stopped transmitting signals. It could very well have been a downed transmitter.
But they had a survivor who had been there 57 years ago, describing a hostile alien and the possibilty that there may be many more. Unfortunately she is suffering from PTSD and there is nothing to coroborate her story.
There "might" be an indigenous hostile organism on LV-426 that the Company did not know about, so they send in a platoon of Marines. Nothing too large so as not to attract too much attention.
The simple possiblity is that the Company grossly underestimated the situation on LV-426. They merely believed communications had been lost, and the colonists may have encountered "large termites" that needed to be cleared out. More of an annoyance than a serious threat.
The Marines complain about having to do "another bug hunt." This probably means that they have done this kind of "house clearing" before, and absolutely hate it as it beneath them (they are Marines, not exterminators). So it is "just another bug hunt." No big deal and they will be back home in no time.
It isn't until it is far too late that they realize they should have brought more men.
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The "complex conspiracy" possibility is that Weyland Yutani DID know how serious the situation could be, and knew they were responsible for all the of the lives of the colonists if the worst case scenario was that they were ALL dead.
So they send in a "small" team of Marines. The smaller the size, the easier they would be to control, and the better off the Company would be should they have to "eliminate" the Marines (and anyone else involved such as Burke and Ripley) to cover the whole mess up.
Remember this is the same company in "ALIEN" who put the profit of collecting the lifeform specimen above the lives of the Nostromo crew.
My 2 cents.
EDIT- Man I hate not having learned how to type quickly. :lol
Kevin