Because it’s easy. Criminals seek the target of least resistance, like how old cars are stolen a lot compared to luxury sedans with a bunch of tracking and security vehicles.
A lot of the shooters are racist towards black Americans but they would never try a mass shooting in some Bloods infested neighborhood because they know the residents will shoot back. Others are racist towards Hispanics but they wouldn’t dare step foot in Juarez to do a mass shooting because they’ll end up headless on a sidewalk. So they go after people who have never caused any problems to anyone
Not to mention, like many serial killers, they’re have delusions of grandeur and believe they’re on a mission that can only be achieved through violence. Not to mention, they also want to achieve maximum shock value. Eviscerating a woman’s guts and leaving her in the sidewalk for joggers to find is old fashion. Targeting old people and elementary school children and live-streaming it is the new normal.
Rich and powerful people rarely get hurt.
This is why leftist terrorism is rare. We have seen it in the past during the Cold War. The RAF killed politicians and fascist leaders and foot soldiers. The Japanese Red Army killed some civilians and hijacked planes, but they were part of a larger political goal that was immediate (I.e. the civilians were Israeli, the plane was going to North Korea). The Italians went on bombing and shooting campaigns during the Years of Lead because fascists targeted them with the help of NATO and the CIA. Most leftist terrorists target actual people with influence and power. That’s why they’re prioritized more than fascists when it comes to repression.
The ugly truth that liberals don’t want to acknowledge is that there are only two ways for gun control to happen in the US. 1, if minorities start to become not only armed, but also resistant to the American institution. The conservative minorities who own guns don’t do squat to threaten any power. Or 2, rich people and their children are targeted by mass shooters. The rich will always prioritize themselves so if things get out of hand, they have no problem violating any gun rights. 2 will be unlikely to happen, but 1 is quite literally how gun control came to existence in the US, so it might happen again.