Because you are too stupid to realize that staying in touch with your mom doesn't make you a "mamma's boy"?
Your parents are family. They support you throughout your life and are there for you when you need them. The least you can do is be a decent son and stay in touch.
Now, if your parents are abusive and mean and they don't support you in any way at all, THEN I can see why someone would want to avoid contact with their parents and not want anything to do with them. But, as it stands, I have no reason to make that assumption. If YOU are dealing with such a situation then I'm sorry because that's awful and you don't deserve that.