Jonathan Gruber describes Obamacare, circa 2012

What’s important to remember politically about this is if you’re a state and you don’t set up an exchange, that means your citizens don’t get their tax credits — but your citizens still pay the taxes that support this bill. So you’re essentially saying [to] your citizens you’re going to pay all the taxes to help all the other states in the country. I hope that that’s a blatant enough political reality that states will get their act together and realize there are billions of dollars at stake here in setting up these exchanges.

There is more detail here, from Peter Suderman, along with the video and also the fuller context for those (such as myself) who have not been following this issue very closely.