It's obviously because:
a) it's intensely political and thus soon becomes tribal, whether or not a person is in a tribe or making a tribal point
b) they may have the view that "the conditions Black people face, especially in America", which we are all going to conclude that you mean institutional racism, is not correct and thus not something they will put their time into
Thus, it is too much trouble for them to get involved in and not something they believe is worth their time. I can even imagine attracting opprobrium for this comment, which is ironic.
So, as with all such things, try not to daemonise your opponents (send to the background to eventually die?:) and allow people to disagree. Take people's points as if they are sincerely made and without malice, accept that they may have a point. Even your question implies that people are making choices based on an irrational or ignorant bias. Cut it out and you may make inroads, which would be a good thing, right?
In short, apply the principle of charity[1].
[1] https://en.wikipedia.org/wiki/Principle_of_charity
