...because that's what the law currently states? I mean if you want to argue that it should be legal for landlords to discriminate based on religion, race, or gender, go right ahead... but good luck with that, as I don't think there are any states left in the Union that allow that sort of thing, nor very many people left who think it should be legal to do so.
There's plenty of people who think government should stay out of it. If, for example, a Mexican restaurant wanted to do Mexican-only Friday nights to celebrate their heritage and feel at home, there's nothing inherently wrong with that. Of course, that's not remotely the full story of what would happen were the anti-discrimination laws lifted, and I don't mean to advocate anything. But for most issues there are valid reasons to take either side, and there's a lot of people who can express them if you get them to open up.