In the Alien universe mega-corps like Weyland-Yutani basically own the government lock, stock and barrel. The Colonial Marines are only nominally government run, most of their funding comes from fees paid by the mega-corps for handling issues that their own PMC's couldn't. Basically, Weyland-Yutani downplayed the situation so minimal troops were sent to investigate, pulled strings to get Burke/Ripley aboard and made sure that the officer in charge was "controllable". The Sulaco was largely automated due to being an outdated, disposable design. The whole op was being run on the cheap because that is what the Corporation would pay for and they wanted to keep things "low key" in case they could make a profit from what was otherwise looking like a major disaster.