Could be that Carrie STILL wasn't really sure she wanted to spend the rest of her life with Big after all. And really, if you've got a happy Carrie in a committed relationship, there's no reason for the SATC universe to continue to exist- there's literally no place else for the characters to go, particulary if the organizing principle of the whole thing is about placing Carrie and her self-centeredness above anything and everything else in the structure.