You can always check the config from Chocolate, the mod that spawns those cities and dungeons, in %appdata% -> Roaming -> .technic -> modpacks -> hexxit -> Chocolate -> Dungeonconfig
I don't know how to open those files, but if you're familiar with configs you should find the answer there.
My guess is that Technic might have reduced the spawn of Nethercities to zero, just like the volcano dungeon.