After everything we know about most mass shootings, why do they automatically blame the AR-15?

Because of the cosmetics ,it scares them. Its their focal point on their hysterical journey deep in fantasy fear.

It's a mean looking gun it's scary. Put one side by side with a Mini 14 ranch and a non gun person will point to the AR every time as being more deadly

Because TOO many believe every thing that they hear (IF it is from those that they like to listen too).

Because it's much easier to place blame on a tool than it is to place blame on a person.

Agenda.....

In my opinion, it's ignorance, political correctness, and emotional instability.
The are so many people that don't know the AR-15 is NOT and automatic weapon. We hear, all too frequently. The coupling of "machine guns" with the AR-15. The newest moniker is "weapons of war". Despite the fact that our military does fire semi-auto in combat much more frequently than 3 round burst, the fact remains the AR-15 is strictly a semi-automatic rifle just like many modern hunting rifles and shotguns. We can blame the media and politicians for spreading this ignorance.
We already know how defensive people get when you blame a Muslim, mentally ill person, or any minority for any given offense. Instead, they must deflect the truth and attack the tools and the "unprotected "classes; most notably, the white, conservative, Christian, gun owning male. The mass shootings will continue until they stop hiding from the truth.
Finally, the emotional side of things. Admittedly emotional instability is not really that accurate of term. It was perhaps an aggressive way of saying that people react on emotion after a mass shooting. This leads to irrational decisions culminating in knee jerk reactions leading to simplistic solutions that fail to solve anything. All they do is cry "assault weapons. Boo Hoo" without out taking into account the small number of deaths actually committed with these weapons, ay least in comparison to the total deaths of which most are committed by gangs of which the two largest groups represented in gangs respectively are Hispanics and Blacks. Se where I'm going with that?

Because it's black. Because it's scary. Because it can shoot a billion rounds in only 3 seconds.

The actual answer is people are afraid of what they don't understand. If my knowledge is correct, I think the AR-15 is probably the "biggest" "scariest" gun available to the general public. People don't understand that one trigger pull releases one round which potentially kills one person. It doesn't matter if it's the biggest rifle or the smallest pistol.

Unfortunately, people don't want to educate themselves on the subject. They would rather demonize "mass killing machines" without taking into consideration the everyday items they use which have a much higher body count than a firearm.