Basically, when there are a ton of zombie pigmen, blazes, and/or ghasts in an area of a nether fortress, nether fortress mobs, like skeletons and wither skeletons will not spawn unless you kill a lot of the other mobs. This makes it very difficult to farm for wither skeleton skulls without either manually killing the other mobs or using /kill @e[type=____].
What I suspect is happening is that there is a max monster limit in an area and all of the pigmen and ghasts are taking the positions of the wither skeletons and regular skeletons that spawn in nether fortresses.
How to replicate:
Go to a nether fortress.
Kill all of the spawned wither skeletons
To see that they are not spawning do /say @e[type=wither_skeleton]
Run around the fortress and keep typing that command.
Once the entire fortress has been generated and all of the skeletons have been killed, they will no longer spawn unless the other mobs are despawned and/or killed.
Possible Solution:
Give wither skeletons, skeletons and magma slimes their own "place holders" in the mob spawning system so that they will spawn even when the pigmen and ghasts are spawned. By having seperate spawn caps for both sets of mobs, both sets of mobs can spawn freely without one dominating over the other (pigmen not letting other mobs spawn).
This seems like an issue for 1.15.1 too. I spent nearly and hour in my nether fortress on hard mode, and found only 12 wither skeletons. Refreshing the mob spawns by switching to peaceful and then back to hard seemed to help, indicating that the pigmen are taking up the spawning spaces. The fortress is totally empty except for some pigmen and blazes from the spawners, while my nether fortress in pocket edition is totally full of nether fortress mobs.