Because this is not a theocracy and if he has his doors open for business he can't discriminate. We can go on but once discrimination is allowed for businesses , there will be no end.
Go to church if you want religion, I am big time in separation of church and state, and you zealot Christians think for some reason Church means Christianity and it does not. I want freedom from religion in public and in business, religion is for home and church.
I'm not a zealot Christian. My interest is more in law. I understand though that Jude 1 and Romans 1 is the "law" of Christians forbidding them from enabling the spread of homosexuality in a culture using the important social vehicles or icons of marriage, education or adoption. If they fail to stop enabling that spread, the Bible, their law, is clear that God will punish their immortal soul for eternity.
I think it is important in the OP for everyone to understand the commandment of Jude 1. It is not a commandment to hate individual sinners, but the sin and more importantly facilitating the spread of the sin itself using "smooth speech, manipulation" and social icons to do so. Folks need to understand the Christian objection to get why Christian bakers may actually love their gay clients but CANNOT bake them a gay wedding cake, no matter what. And, you can't force a Christian to choose between eternal soul death or going out of business.