If each state has its own little rules, then it is hard to prepare for the fight. If it is so hard to have a one single rule for knee to the head, why not simply forbit it? Allow only knees to the body or jumping knees. Then there wont be such cases like with Evloev, Walker, Aljo. Btw, I have recently found out that in PFL, any kind of elbow strikes are illegal. Just any, not controversial 6-12 elbow, but every elbow is illegal. This narrows fighters arsenal, but it is simply to follow that rule. If PFL managed to have such rule, then no knee to the head can be introduced in UFC easily.