Because unlike same sex marriage circumvention and legislation by way of the judiciary ala the 14th Amendment which guarantees civil rights to all and where marriage although religious in nature is not religious necessarily; religious association is quite a different thing, and is specific to the state of mind and covenant with anyone's God, independent from state interference. In other words, marriage is religious in nature only, but legally requires a license to procure, thus state direct involvement, however, a business owner has no such impediment under the law, where competing liberties are at odds.
It really comes down to that. States are exerting specific protections for competing liberties, and providing remedies in anticipation or preemption of challenges by gays, or anyone else that contradicts religions and conscientious objectors.
That about sums it up, and it only took 200 plus pages to get here.
You're all welcome.