I don't see how it's so peculiar - its mainly the evangelical hardcore that still hold that torch and they represent about a third of Republicans.
I honestly wonder whether SSM would have a chance of passing in a referendum even in the most conservative of states.
2012 is the year it felt like same-sex marriage had majority support in the country, and 2015 is when it was legalized everywhere. After so many years it can be hard to hold out opposition on this issue.
I think 2012 is the year when the Democrats realised the issue wad not necessarily a losing issue - Biden and Obama endorsing and still being re-elected and four red states either legalising by popular vote or rejecting a constitutional ban.