Why do many Americans feel a distinguished sense of racism towards Persians(Iranians)?

It's called ignorance, really. It pisses me off and I'm black, I'm not even part of that race/religion/culture. I think it is so stupid to generalize that all Iranians are terrorist because of 9/11. But it's funny...That some Americans are quick to call them terrorist, yet no one brings up the Oklahoma City Bombing when two Christian and American terrorist killed over 100 people. Oh, but no. Not all Americans are terrorist because of this right? Of course not but when someone that isn't American does it, everyone belonging to that group is a terrorist. It sickens me.