insurance companies didn't take them up on it, for various financial reasons and issue setting up network.
This is an area that has been left to the states, and you have different laws.
to have many more insurers selling across state lines, you'd likely need to have the laws the same in the states - ie. federalize the whole system
http://www.rwjf.org/content/dam/farm/reports/issue_briefs/2012/rwjf401409