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.
^ Yes, it will be easy for the fighters to follow simple rules. Because in the heat of the moment, it is very easy to actually do something that you are not supposed to. So I also think that the rules like 90-degree elbow or shit like that should actually not exist. Either he can throw and connect elbow or he can't. There should not be any in-between. But we don't make the rules so the fighters will have to actually follow the rules. The sad part is we have seen a lot of fighters lose the belt because of these rules. And many of the times if not all, they were unintentional.