There's no way the inhabitants of this so lovely dungeon have hatred against their neighbour! But they can have a lot of motives to stay on their floor and not meet each other. Here's your plate of reasons you can sample as you wish!
The dungeon finances are limited
The dungeon lord, who's at the bottom floor, may have limited resources to spend, and so much employees to take care of! Hence, in the contract he has signed with them, their protection task includes only one floor. Then, why, as a truly evil employee, would you ever thinking of working more than what you're paid for?
Also, since the dungeon lord is evil and very, very stingy, they want to "fire" the less efficient employees by putting them to the more "risk-inducing workplaces", which are obviously at the top level of the dungeon, where the "clients" are many and meanies. But the employees may be stupid but they're not fool and understood what their boss did. Therefore, why should you pick others' dangerous slack when you can have a peaceful nap just below instead? No, no. You're not paid enough innocent souls for that!
They want to live in their favored environment
Each floor has been carefully made to allow specific species to live in. 1st floor has woodland inhabitants, 2nd has cavernal creatures, and so on.
The motive to go to another floor becomes basically void, since other floors offer nothing suitable to sustain their prefered lifestyle. While you don't hate your neighbours, you don't really care what happens to them, since you don't meet them on a regular basis.
Here's a more flavorful bite of this sample with ice ogres and lava snakes : they won't even dare venturing in each other floors, since the first one will melt on the hot smokey fire caverns while the seconds will freeze to death in the cold, lifeless chasms.
They don't want to step on their friends' traps
Monsters are sneaky ones. And paranoid, on top of that. They have laid traps of their own design in everything that looks like a room, a door or a chest. And they have so much problems trusting each other they won't tell anyone but their floor's comrades. And because of that, monsters of other floors can't simply rush in to help without falling in them.
After all, how can you prove you won't betray us as soon as things go awry in order to earn a getaway or a good place in the commandment? What tells us you won't change the traps so we fall in them, just to joke on us? And more importantly, aren't you actually working for the adventurers?!
Pushed to the extreme, small groups of monsters may band up together in some rooms, and they wouldn't know anything of the traps their neighbours on the same floor laid! They wouldn't help them from fear of being tricked or simply because they can touch the wrong pressure plate!
They follow strict caste rules
Monsters can be ugly, nasty and love to have some human 'hors d'oeuvre' at dinner, but they do have a strong sense of order in their life. And this reflects back to how the monster society is organized.
And it is organized in classes, where the beggars live on the cold, dangerous top floor, while the royalty live at the cozy and warm bottom. No beggar would ever think of getting near nobles, that's an offence to honour, and is a proof of disrespect against your leaders. At the same time and more importantly, most nobles don't really care about what happens to the lower blood, except for the rare few who care deeply for their people who look with sparkling eyes at them!
Hope you enjoyed the samples! Know that you can easily mix in the ingredients I gave you to cook whole new flavors!