Why did everyone on the Sulaco go down to LV-426?

Remember, the real goal was not to investigate what was happening, save anyone, or fight the xenomorphs. In Aliens, as in Alien, they wanted to bring alien samples back alive. That is what Burke is there for.

Since he's meaning to gull everyone else on the mission, killing them if need be, Burke wants everyone as close as possible to his domain of control. He doesn't want an independent group of people making decisions without his immediate presence. When he tries to get back on to the orbiter with alien organisms in tow, he doesn't want anything to resist his entry and return to civilization. (Remember, Ripley wanted to keep the alien out in the first movie.). Ultimately, it boils down to Burke (and his bosses) deciding that was the best way for him to control the situation.

It might help if you think of the Sulaco and the dropship as a single unit. The Sulaco doesn't need a separate crew since it's just being used as a glorified transport vessel, to get Gorman's military unit from A (Earth) to B (LV-426). The comms officer, the pilots and the science officer that fly the ship are all part of that same military unit.

As such, there's no useful purpose in having extra personnel set aside simply to remain on the Sulaco. It's not clear what they'd even do while the military unit is doing their thing down below and in the very unlikely event that they need something from the ship, the ground troops can just travel back up to the Sulaco and load it with only a few minutes notice. They can use the remote uplink (or the colony's satellite) to communicate with Earth if an extreme need arises and the spare dropship can be remotely piloted by Ash if (as happens) the first dropship is disabled and pilots are incapacitated.

In short, lugging around extra people makes no good sense, either militarily or financially.

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)