Why does my mother act like this?

My girlfriend and I have been together for about 2 yrs. and her and my mom have NEVER gotten along. The reason they don't get along is because my gf is black (dominican and puerto rican) and according to my mother "all black people are bad people". Over these past couple of years I have done everything in my power to try and prove to my mom what a beautiful and wonderful person my gf is, but she just doesn't want to hear it. Now my gf is 5 mos. pregnant and we told my mom a few weeks ago, she wasn't happy about it and went as far as calling my child a "n*gger baby". I wasn't having that so I blew up and ended up cussing my mom out. Now apparently she went and told the whole family what I said to her and now everyone is calling me and telling me how wrong I am and how I should apologize to her (which I don't plan on doing anytime soon). So I'm assuming she didn't tell them the truth about what happened. Why is she like this? How can somebody have so much hate for a person that they barely even know?