Why do people care about racism so much?

I wrote in my profile that I am home schooled. And now people are making fun of me because of that. They wrote "you are homeschooled, that explains it." Now what if I were to write to some black person "You are black, now that explains it." And I would guaranteed be bombarded by thumbs down. While the person who made fun of me because I was home schooled got 3 thumbs up. Why do you think that one kind of discrimination is bad, but another is perfectly fine.