Pure and simple casuistry. You're choosing one highly irregular example and applying it as a general rule.
Really? Because there are numerous occurrences of mass stabbings. They happen frequently every year but aren't reported on that often.
Here's one from this year that was luckily stopped before it became worse than it did: Gun Carrying Man Stops Stabbing Spree in Utah - Leah Barkoukis
The point isn't how rare mass shootings or stabbings occur or how likely they are to occur given the commonality of various types of skills sets to use various weapons. Of course fewer people practice kenjutsu than marksmanship. The point is to demonstrate that if a determined killer wants to train with a type of weapon, there are systems of skills that have manifested around essentially anything that can be used to kill people and can be effectively implemented with the correct game plan and determination. The most common element in all these mass murders that have higher body counts is that they managed to pick a place where people are either unable or unwilling to resist. That is the single most important variable.