Of those, only the original trilogy got to keep their own domains:
Stack Overflow,
Server Fault, and
Super User. The other sites with their own domains are special in some way - such as
Ask Ubuntu, which is a sorta official Ubuntu site;
MathOverflow, which has a long complicated history and isn't fully owned by SE; and [stackapps.se], which isn't a regular site - it's for creating scripts and bots etc on SE.