The vast majority of homicides are either situations where the victim and perpetrator know each other OR it's relatively small scale. Random murders are uncommon, and it's rare to see someone who commits a lot of random murders.

Mass shootings get attention because they are both seemingly random and they are often mass casualty events. The methods of many perpetrators of mass shootings often resemble terrorism more than what we would consider crime- lots of victims, no intent to escape, motives that are often irrational.

If the perperators of mass shootings were using pistols or other weapons- that would be the focus of legislation. When Columbine happened and other shootings of the 90s took place involving the TEC-9, the pistol was banned by several states (California banned it both by name, and by it's various features). Same deal with the Hi-Point Carbine, which was also used in Columbine- California, Connecticut and New York have all restricted the sale of the weapon at various times.

The AR-15 is what many high profile mass shooters are using, so it is the focus of the legislation. Unlike in the 90s where restrictive legislation passed in many states, it seems like the failure to pass legislation to restrict the AR-15 has caused a loop where more people buy them and then odds of a mass shooter using an AR-15 instead of something else are higher.