Based on the fact that interracial marriage and inter-tribe marriage has always been around, and was only banned for a few decades as a reaction to the whole slavery fuckup on our countries part. It was also partial, as it didn't include other races, just whites and black.
Same sex marriage is a concept only 20 years or so old, and thus has no establishment in history, and thus is a new concept that can only be added to our laws via legislative action.
As States issue marriage licenses, issuance is a State issue. However as recognition of the licenses is a Federal issue, and full faith and credit is part of the constitution, States cannot be forced to ISSUE same sex marriage licenses, but they can be forced to RECOGNIZE valid same sex marriage licenses from other States that do issue them.