Why what? Why people would not like Kap? Well yes, some people are racist. I hate to break it to people here, but some people are racist.
Others just don't give a damn. They don't like racism, but they also do not like being reminded of it.
Others do not like the break from tradition. Athletes are to be seen and not heard.
Others just love to complain.
Here is a real-life Lap example, Muhammad Ali became totally reviled for refusing to serve in Vietnam. He was hated. But as time went by, Americans came to accept Ali again. Sure, the racists never did, but the "do not bother me with your painful truth" people came back to Ali.
The "do not bother me with your painful truth" people would have hated anything Kap did. Like the made up quote attributed to Churchill, you can count on Americans to do the right thing after they have exhausted every other possibility. We were not ready to admit racism, we did not want to admit racism still exists. Kap confronted us with that, it was not going to be popular.
And to offer up a newer study on race
https://www.theroot.com/a-judge-asked-harvard-to-find-out-why-so-many-black-peo-1845017462