There are several answers, but it depends on the dungeon and the monster.
Take, for example, the puzzles. Because no campaigns I know of start at the bottom of a dungeon, we can turn to reality. In tombs like the Egyptian pyramids and Chinese Emperor burial sites, the pallbearers, in order to keep the secrets of the traps, were trapped in themselves. This means that, logically, the traps work both ways. Therefore, monsters wouldn't push up of they were monsters more muscle than brain and couldn't figure the traps out.
The more intelligent could face physical barriers. For example, some are foiled by a locked door, no more. If they lack the faculties to break it down, then they cannot move on. Another possibility would be that moving up could cause harm- of they are susceptible to physical attacks, staying where they are could be safest, and they would be smart enough to realise it.
So that addresses the less intelligent monsters, but what about the bosses, intelligent and strong?
Bosses stay where they are because they have a purpose. Many times the bosses are set personally by the Dungeon Creator,and this understand why they should stay. Sometimes it is the creator itself. So, the boss knows why it needs to stay where it is.
Of course, this is all overlooking, one other thought- perhaps the monsters were spawned there. Let me frame it this way: if you were born in an opaque dome with no outside contact, and you were told that there was no outside, would you try to escape? No! So, it is possible that the monsters simply do not know that there are different floors to move to.
Overall, they could be unable to overcome a trap, or weak, or understanding of a purpose, or just kept on the dark.