God told him to do something that instead of appearing to help the people he was trying to rescue was making the situation worse for the moment and Moses appeared to be a hoax causing them more trouble. They were stubborn because they wanted immediate results. The situation was tough and they could not see the end. I think it was mostly unbelief. They could not see any headway being made. I am in the same situation right now. I want to have faith and believe all will be taken care of by God, but each day appears to get worse and worse!
God had to command Moses and Aaron because they were arguing with him and trying to change his mind to act in the way they thought it should be handled. So, God had to give a direct command.