Because legislating every conceivable fear and every instance of mass panic will lead to an overly complex and overly burdensome legal system that hampers legitimate activity.

In the words of Cicero, "more laws, less justice"

When we start having problems with net neutrality, we can legislate against it. As we don't have any such issues, and there is no evidence that we are in imminent threat of net neutrality being dismantled, I see no reason for net neutrality legislation.