Why Fiends rarely enter the prime material plane overtly:

[jester47]

Simply put: It is dragon territory.
And dragons, by and large, good or evil, lawful or chaotic, HATE fiends.

 

[Michael Silverbane]

Also, fiends are lazy, and don't like to travel.

Later
silver

 

[Sejs]

Why Fiends rarely enter the prime material plane overtly:

Paladins wandering all over the place make the whole plane like one big minefield.

And if you operate openly, they start coming to you in droves, and they don't stop coming until you're dead, banished, or sealed in somebody's basement for a thousand years.

I swear, there's just unending hordes of the the things! It's a nightmare for any right-thinking, upstanding fiend! It's horrible, I tell you! Horrible!

 

[Whizbang Dustyboots]

Also, fiends are lazy, and don't like to travel.

Not to mention the hassles with passports and ticket prices, and everyone knows when you go eat or drink on the Prime Material, you end up with Mortal's Revenge half the time, and no one enjoys being around a fiend with gastrointestinal distress, even other fiends.

 

[William Ronald]

Also, add the mortal worshippers of deities who can and will call on their diety for assistance. If a fiend is not careful, some cleric may decide to gate in a few devas or a few solars. Also, whether or not one uses the Blood War, the various fiends have a lot of machinations and wars among themselves. If they spend too much time focusing on the Prime Material Plane, they may find themselves having to deal with trouble back home. So, this is perhaps why fiends focus on corrupting mortals as opposed to large scale planar invasions. They can pick up a few souls for little risk, while avoiding big problems. (Invading a prime material plane world might unite every hostile force, so even the worshippers of evil deities might be battling fiends attacking their world.)

 

[Sejs]

... If they spend too much time focusing on the Prime Material Plane, they may find themselves having to deal with trouble back home. So, this is perhaps why fiends focus on corrupting mortals as opposed to large scale planar invasions. They can pick up a few souls for little risk, while avoiding big problems.

Yeah, just imagine how embarassing that would be. You come back from a really kickass invasion on the prime only to find someone's changed all the locks on your bastion of evil and there's a foreclosure sign on the front door. Your girlfriend the succubus has hooked up with a half-dozen other guys. And there are some heavies from the local archfiend trying to get a hold of you over something about 'missed payments'.

 

[Glyfair]

Plus, in the early days they typically weren't supposed to enter the Prime unless invited or summoned. Occasionally they'd find a week link in the planes and slip in, but for the most part armies of the abyss couldn't just invade.

 

[Shemeska]

"They have better things to do. But don't worry mortals, you'll be gotten to eventually."

And now back to OOC...

Most of the fiends that can make the transit to the Prime by themselves, generally do have more pressing things to worry about on the planes. Typically it's more damaging to them to just abandon those commitments, be they to the Blood War, or internal struggles in the Abyss, Baator, Gray Waste, Gehenna, or Carceri. Sure it might be fun for a Pit Fiend to abandon his post in Baator or as a marshal of a pack of a hundred thousand barbazu marching across Oinos, all for what amounts to a free buffet on prime material world X, but he'll probably be demoted to lemure status for that little stunt.

For fiends that can't make the transit on their own, they probably have less to risk if they do manage to find a stable portal, or end up being summoned and slipping their bonds. They'll have fun before they get shunted back to their native plane certainly.

Of course, sometimes the Blood War itself spills over onto the prime, probably just as often by horrific accident than by intent. When it does, it normally ends up sterilizing that world completely and probably adding some new real estate drifting in the Astral, IYKWIMAITYD, or at the least leaving it a burned out, ravaged husk that will never be the same.

 

[Whizbang Dustyboots]

I know this is a simple question with a complex answer, but what's so pressing on the Lower Planes that they'd want to stick around there for? Isn't one fortress made out of skulls and mortared together with the congealed bodily fluids of the damned pretty much like another?

 

[Terwox]

I know this is a simple question with a complex answer, but what's so pressing on the Lower Planes that they'd want to stick around there for? Isn't one fortress made out of skulls and mortared together with the congealed bodily fluids of the damned pretty much like another?

No, only one of those estates is the one you built with the broken bodies of your slain enemies over the course of an aeon. Anywhere else might be nicer, have more screaming prisoners, etc, but it's just not HOME.