It should also be noted that the data set they used only include mass killing with guns, and not mass stabbings. Those also happens, including here in Sweden a year ago at a school. But if you don't like that data then bring more and we can discuss.

https://www.cdc.gov/nchs/pressroom/sosmap/Firearm.htm

Two things pops out. First that the variance is massive within the united states. The second part is that 63 percent were suicides. It is very doubtful that a single correlating factor accounts for the variance.