It's also much easier to defend oneself against a knife attack in many cases. E.g. pick up chair, hold in between you and knifey guy; not exactly a great situation, but he's going to have to spend significant time and effort on each and every victim he goes after, and slowing things down greatly will dramatically reduce the body count.
Knives are simply much slower and less efficient for the task, especially against forewarned opponents (e.g. most of the potential victims in many mass-killing situations, and potential defenders), and it's vastly more likely the attacker will be stopped quickly even if he manages to kill a few people.