How did most people come to be on Pandora?

Tannis gives you some hints to this when she tells you the history of Dahl and Atlas and the like.

The bandits you fight out there used to be family men,
workers, scientists... We're all broken because of them.

These big corporations brought people in, then just abandoned them. It's easy to get to Pandora when your company deploys you there, and much harder to get off the place when you have to pay your own passage. Spaceships aren't cheap.

The low number of peaceful people compared to psychos is, most likely, a game mechanic. I.e. the game is a shooter, so it needs hordes or respawning enemies, and pretty much doesn't need any civilians beside Marcus, so there are just a few.

The line between NPCs and enemies is more blurry than "real humans or not" - Marauders are human, you can't even call Midgets or Bruisers non-human. The world of Pandora is violent, there are many factions constantly at war, plus every other person is crazy or mutated - so it just takes a little of that shooter gameplay requirement to turn the whole planet into a perpetual bloodbath the game is.

Many people arrive on Pandora - some with corporations, some on their own hunt for glory or flight from the law - like most vault hunters. But other people live on Pandora for generations - for example, Salvador is a native.