The difference is that from 1066 until recently England has had one dominant ethnocultural group. Relatively small groups of immigrants are able to integrate into this culture seamlessly.
Black people in England aren't culturally "Black", they are either culturally English, or culturally Trinidadan, Jamaican, Ugandan, and so on. Or some combination/superposition of the above.
There is no particularly strong shared experience between a Trinidadan-English person and a Zimbabwean-English person other than living in England.
Blacks in the US, on the other hand, have a common and separate culture. 20th century immigrants are a tiny proportion of the (biologically) Black population of the US. The overwhelming majority of US black people are descended from slaves.
This population has been separated from the white population to varying degrees from the time of slavery to the present day. Now there is more mixing so the cultures are converging, but that can't happen overnight.
You may as well ask why Scotland and England have separate cultures, or England and China. It's not a matter of skin color; it's a matter of a group of people that have primarily had contact with each other, and less contact with other groups, for hundreds of years. I am not sure how else to explain it.
Edit: re-reading your comment. I didn't realize you were asking about communication difficulties specifically. This can probably be largely explained by the fact that African-Americans speak their own dialect of English that is in some ways quite dramatically different from the (White) standard. Here is some info: https://en.wikipedia.org/wiki/African_American_Vernacular_En...