Because white supremacy as major force in this country is a joke. You're talking about a population size that's so small (and spread out), insignificant, and frowned upon that their numbers can't even swing a small town election with a population size of 1000 or less. When there are rallies, there rallies, at best, might get to 50 people. On rare occasions, they may be around 100 people (but even that's pushing it). They have no influence in society, have no real power, and you only see them in their small rallies. That's it. They're that pathetic. Do you have the occasional deranged person who is a white supremacist? Yes. Are they the only people who go on killing sprees? No. Chicago and Baltimore have gang problems every week, and it's typically black on black violence, with the occasional black on white, white on black, and white on white violence. Whenever an Arab goes nuts, he's usually a Muslim that follows Islam's fundamental teachings. Of course you have Hispanics, typically a bunch of illegal aliens, who murder, hurt, or rape others. What makes this even more interesting is that the Left says these white supremacist killers are alt-Right people, but whenever a manifesto or a couple of social media posts pop up, you find out these people are 99.9-100% (can't be any lower) of the time a bunch of Leftists.
The El Paso shooter sounded like environmental, anti-immigrant anarchist, while the other is a Satanic, socialist believing (would-be) rapist. Both were down with the Leftist agenda, one REALLY liked Crazy Bernie and Fauxahontas Warren, one didn't like Hispanics, and both come from broken families (one of the Left's favorite targets). The guy in the New Zealand shooting had a very similar background. In his manifesto, he believed in man made climate change, population control, communism/anarchy, racist against Arabs, didn't like Trump, and was an atheist (although there are Right wing atheists but don't believe the other stuff this guy did nor are they as violent). Sounds like a typical Leftist to me instead of someone on the Right or Middle. Even alt-Right people sound like they're Leftists. Although it's not white supremacy per se, but let's not forget the guy who shot up a baseball field filled with Republican Congressmen because he hated Republicans (managed to send one to the hospital in critical condition). He also was a Crazy Bernie supporter and listened to people like she-male Rachel Meadow, supporting all things Leftist.
If you ask me, white supremacy seems to exist more so on the Left than the Right and Middle. Violence seems to be a trait that these guys carry, too, along with their racist rhetoric. Of course, violence in general just seems to be one of the top traits of the Left. Antifa, Black Lives Matter, the Black Panthers, lone wolf attacks, they all bring rage and hatred with it towards those they disagree with, even if it's their own people. The thing in Charolettesville, it was a gathering of Leftists, with racists on both sides, who didn't like each other and one of them was crazy enough to kill some people with his car. Of course, violence like that happens in Chicago EVERY WEEK. Why do they act like this? Simple: no (good) father figures to guide them, not being taught the differences of reality and fantasy, schools teaching them their parents are evil, politicians saying man-made climate change is real and is going to kill us all in 10 years, and they're being told that they're "special" before even doing anything that makes them special. Eventually, for some, white supremacy becomes a thing for those on the Left.