Why doesn't UK rap crossover to the United States?

This is a question that I constantly ask myself. I started listening to UK rap 3 years ago and at times I find it better to listen to over American music. I'ts interesting because UK pop artists have successfully crossed over, but UK rap artist have not. For example Adele and Ed Sheeran are very popular in the US. Drake has feature UK rappers on some of his latest albums, but those songs are usually the least liked. I don't know what it is , but Americans are missing out on a lot of good music from the UK. Why do you think it is difficult for UK rappers to crossover to the US audience? Who are your favorite UK artists?