For many Americans, travelling out-of-state is a big deal. If its important enough many of them can make it happen, but many seem to have decided that formalizing their relationship in one legal category is not that important.

There are other reasons why Americans might become less likely to choose one legal form for their marriage, like an American friend of the right-libertarian persuasion who does not just object to marriage, he objects that the laws of his state declare that he and his long-term, live-in partner have various obligations to each other which they have not explicitly contracted. OTOH there is the daft way healthcare works in the USA which can create incentives to use a particular legal form.