I doubt the US was one of the MOST racist let us not forget South Africa and all of Europe (the US is still light years ahead of them)....Sri Lanka, and other countries also had and have race issues to this day

But yes it was due to race but also Jack Johnson talked shit, he had a white mistress, he had gold teeth, spent hella money, had no mind for paying attention to how he "should" act, but looked out for #1 made fools of those that fought him and began the era of arrogant fighters though it came well after a Johnson Hangover in the 1920's-1950's

Max Schmelling was another political bad guy...he was in real life a nice dude but he represented NAZI white supremacy and greater power than the US.

Lennox Lewis was far from loved after he beat Frank Bruno