The discussion is about race because a certain faction has made it about race, simple as that. As you correctly point out, this is not an issue restricted to any one particular group - we are merely seeing the worst 0.00029% of interactions with police. (no, literally, 0.00029%, not just a made-up number. Which is to say 987 people killed by police last year out of 340,000,000.) This would seem to reinforce the notion of concept of scale.
Since 88% of all people killed by police last year were armed, that doesn't leave very many who weren't ... precisely 68 people, actually. That would mean according to the 39% figure, you could expect 26 of them to be black. However many of those you say "shouldn't" have been shot but were anyway (in other words, the victims of "disproportionate" violence as opposed to the normal run rate) would be at most 15 of 16. For the entire year, across the entire country. And this is after allowing the HUGE concession that even though you likely know nothing at all about the individual circumstances under which most of these people were shot, we'll go along with it and assume it was because of police bias. Does that look like an indication of systemic racism to you?
https://www.washingtonpost.com/graph...hootings-2017/
For that matter, why would 13% of the population be involved in 37% of the arrests for violent crimes? Uh-oh, wait a second - that almost exactly matches up with the 39% of unarmed people you say are shot without justification.
It's almost as if being involved in a higher number of volatile situations with the police directly corresponds with a higher chance of being shot by the police, in a nearly 1:1 relationship. Maybe even less than that, since according to the official stats, only 22% of the people killed by the police overall (armed or unarmed) are black.
For that matter, why are men 50% of the population but 95% of those killed by police? That's not FAIR. How come gun owners are less than 30% of the population, but almost 90% of police shooting victims are armed? That's not FAIR.
Wait, I'm just being rhetorical, and I know damn well that the reason why percentages aren't the same is probably because those specific individuals were engaged in dangerous behavior at the time? But which one is it, then? That individual behavior makes a difference in the outcome? Or that individual behavior is uniform and any difference in outcome are due to bias? In that case, the activists should be far more concerned about the rampant sexism that men experience at the hands of the police. And if behavior is assumed to be uniform across all groups, shouldn't the police's behavior also therefore be uniform? (Actually, what this means is their entire argument falls apart).
So, back to ...
The reason is because race is an amazing tool for amplifying the importance of something far beyond what it would normally be. You bring up police shootings under normal circumstances, most people go "well yeah, it sucks that people occasionally get shot." Bring up who was shot and where, and you get "yeah, it sucks, but it's kind of common sense to know that more bad shit happens in bad neighborhoods." Say it's ABOUT racism, and suddenly everyone is rushing to take a side. Many people will support you doggedly and blindly just because that's the issue.
In a broader sense, this is really about a problem with what the left has become in the past decade or so. There is literally no way that an intelligent person could believe a large number of the points that they hold dear. It just is not possible. So you are left with four main kinds of people there. You have those who are simply too stupid to know any better, who I feel sorry for. You have those who possess some intelligence but have been manipulated or are following groupthink, who I also feel sorry for, but also hold out some hope that they might come to their senses. You have those who go along with it because they think of themselves as independent renegades / counterculture / fight-the-system types, who I can sometimes respect, although I think many are misguided. And you have those doing the manipulating for their own gain, for whom I have no other word than evil.
At any rate, put all that together, and that's how you get where we are now.