Portability is the word they use which is a gobbledygook word for the problems incurred by changing residency from one state to another. If a policy is written for state A and one moves to state B which has no policy similar to that person's original policy how are all parties satisfied? Also, pre-existing conditions, if they have been met in state A is state B required to honor them? And on and on.
Insurance companies never want to pay and congress always gets in and regulates things worse than they were.
It was a difficult problem in 1960, it has become exponentially worse.