A tautology I suppose, but, because you can't change them. You didn't choose it and it's immutable. Not fair to discriminate against someone for something they can't control. This is, of course, personal opinion of mine.
But also, discriminating against things people can control is really just one party trying to apply soft power over another, and I am generally against it. You might be able to legally say you can't work at this company if you like to go bowling in your free time, but I don't think it would be a right thing to do.
For some jobs, other it may not matter.
In fact, US courts have upheld explicit discrimination against people of higher IQ: https://abcnews.go.com/US/court-oks-barring-high-iqs-cops/st...
Religion is a good example of one... people can and do change religions, but we still shouldn't discriminate based on it. There are other examples as well of things that people can change that shouldn't be discriminated based on.
If someone thinks black people are innately inferior to other races, it's culturally acceptable for me to discriminate against them, but if their god tells them that black people are innately inferior to other races, I'm not? Heavily suspect in my opinion.