Why wasn't Colorado sued for the same reason on the issuance of the marriage licences?
Maybe Colorado was; many states faced challenges in the years leading up to the Supreme Court's ruling that a government's provision of special status and benefits to married couples was subject to the 14th Amendment's guarantee of equal protection.
It's important to keep in mind that the baker makes two principle arguments - one, that he has a First Amendment right to discriminate on the basis of his religious beliefs. That argument, as advanced by a public accommodation, is nonsense and will be disposed of quickly by the Court, as it has since 1964.
The more momentous argument is that because his business - the creation of custom wedding cakes - involves the exercise of some creativity, he should be allowed to decide who will, and will not, receive his services. That's momentous because it has implication for a whole host of service providers. Can a maker of sandwiches discriminate because his delicious double-stacked creations are the result of his passion and artistry?
Here, I predict that the SCOTUS will punt. It's important to keep in mind that the SCOTUS gets to pick and choose the cases it will hear. I think it picked this one because it can uphold the laws prohibiting discrimination in public accommodation from First Amendment claims of religious freedom, while sidestepping the baker's status as an "artist" able to pick and choose his commissions. That's because the baker refused service before even initiating a discussion about the design or message of the cake. His intent to discriminate was manifest without regard to whether he would be forced to practice his "artistry". This fact is the lever for the SCOTUS to rule on the issue it wants to emphasize, while avoiding the issue it prefers to leave to the political process.