Because a significant percentage of people who watch Top Gear don’t know what xenophobia is ,or how to spell it.
I have no idea whether that’s true or not as I’m not a Top Gear viewer but if it is then that surprises me. I would have expected Top Gear particularly appeals to male viewers who are like Clarkson himself, ie, middle-class, affluent, and educated.
Some of his “amusing” racist/bigoted undertones appear to be too subtle for the less well-educated like myself. I didn’t previously know what the term “slope” meant in the context which he used it. I certainly don’t recall hearing anyone use it.