I think Kirby set up the idea of spectacular beings becoming advanced to the point of not wanting to deal with conflict implicitly. Attilan, Wakanda, the Eternal's Olympia, and Atlantis exemplified this. The lines blur sometimes, but mutants were different in that they just appeared and we could see how they would end up growing in number to the point that they would follow in the footsteps of the Inhumans because that's exactly what they're doing. The only difference is that everyone knows they exist because they were striving not to isolate themselves and to live in harmony with humans. Also, humans are going to continue to produce more mutants, so it's not self contained like the other cities because humans couldn't produce them randomly.
The Inhumanity event expanded on this when they said that not everyone living in Attilan agreed with the politics of Attilan and decided to either mix with humans or create other hidden cities. I guess this will happen sooner or later with X-Men stories.
The best thing about the inhumans was that writers took a lot of chances with them and used their stories that made them less monotone. Like they went from isolationists, to realizing their Attilan project wasn't going to work because of dissenting opinions causing some to leave and other to attack.
The whole premise for them was "isolationists living in peace in their society" so all of their stories had to push back against them being isolationists (Paul Jenkins, Silent War, Inhumanity) and the fall of their society due to internal or external conflict (By Right of Birth, their other stories).
Since the X-Men are now "isolationists living in peace in their society" they're having stories like AXE which is pushing back on the concept of them being isolationists. Them terraforming Mars is akin to War of Kings.