Why has the AR-15 been the gun of choice for human massacres by a single assailant?
First of all, It’s not. The gun of choice is handguns. AR-15 stories get more clicks, though, so those are the ones hyped up by the media. The vast majority of “mass shootings” as defined by the Gun Violence Archive use handguns.
Second of all, it’s the number one selling rifle style in America. It’s very popular. Using your perception of it as being a “gun of choice” is like saying that “lead is the bullet of choice”.