But they are.
Reed and Sue aren't saying the other kids aren't important. It's not like they cruelly cast them aside. They just let them continue on having adventures.
If anything, it's clear that Franklin would rather be still out there with the rest of the FF kids instead of being stuck at home.
The real world explanation for only Franklin and Val staying with Reed and Sue, of course, is that keeping all the other kids in the cast would make for a very unwieldly book. And also that, surely, it was already known before the current run launched that the rest of the FF would be getting their own title eventually.