Why did Hannity do my homie Common dirty by calling him a "gangsta rappa"?

Because he's a rapper. It doesn't matter who you are, if you're a rapper then fox news automatically labels you a "gangsta". If Justin Bieber becomes a rapper, they might just accuse him of being a "gangsta". It has nothing to do with Common, it centers around fox news' hatred towards anything relevant in the hip hop community.