Generally if a person cannot control that aspect of their identity then it is considered more nasty to attack that characteristic.

For instance you chose the way you dress, what you consume, how much you exercise. Ultimately if you are fat or fit for the vast majority of people it is a choice.

Your ethnic group was determined by your ancestors. You can get more tanned, colour your hair, dress and talk differently, however at the end of the day your genetic heritage isn't a choice.

Religion due to the way ideas latch in the mind is at the crossroads between heritage and choice.

Attacking someone based on something that is not a choice of theirs is fundamentally unfair. Insulting an entire group because of the actions of a few is over simplified, base thinking and uncivilized. So to attack someone based on which ethnic group they inherited is a very low act not worthy of a modern citizen.