It is made abundantly clear that everyone on earth underestimates the menace of the Aliens.
This is shown in several occasions from the way the corporation receives Ripley’s story, to the attitude of the crew or the size of the crew itself. Not to mention the fact that they include a civilian and a bureaucrat in the party. They simply are not evaluating the threat to be high. The story grows darker and more sinister as it evolves, drawing the marines slowly into realizing where they’ve gotten themselves into.
Tactically, given the crew size, with a fully capable and armored jump ship and the ability to remote control another one, it would make little sense to leave anyone behind. The advantages are very few and with a reduce crew it makes sense to have the whole team with you. You’re thinking of it in retrospective based on the outcome but from their point of view, bases on what they know, it makes complete sense. There’s just no benefit on leaving anyone behind.
It would have made a lot more sense to use the Sulaco as a command centre if there were several units that would act on different places or there was the need for strategic deployment and control from orbit over a more serious threat or even if nuke from orbit was an option considered in advanced (it’s only considered after, when they realize the actual threat level)