http://www.cnn.com/2016/06/13/health/mass-shootings-in-america-in-charts-and-graphs-trnd/

136 mass shootings in the U.S.A. in this year alone (depending on how you define a mass shooting), 160 incidents of an active shooter from 2000 to 2013. Your source mentions only 4 mass shootings in which an AR-15 style rifle was used recently. That doesn't appear to make it a weapon of choice for mass murderers.
Also your source, specifically Phillip Carter, mentions that the M4/M16/AR-15 were designed for light recoil so that the shooter would be better able to stay on target. Such a firearm would be favorable for home and self-defense. Here I think you're confusing the AR-15 style rifle with the AK-47.
https://www.washingtonpost.com/graphics/national/mass-shootings-in-America/
Take a look a the chart depicting the firearms used in mass shootings and count the number of Ar-15 style rifles versus all the others. The AR-15 style rifle is a minority. Handguns, however, are the major firearm shown.