There are several difficulties with consequentialism as a basis for moral reasoning. The most obvious is that we do not necessarily know what the outcomes of our choices will be. For example, imagine that I see a young child chasing a soccer ball into the street and save him from being run over by a car. The child then grows up to become a mass murderer. Was it wrong for me to save the child?
Another problem is that the same act may have different moral values depending on our intentions. Imagine again, the case of saving the child from being run over. If I see a child in danger and save the child, most people would agree that I am doing a moral act. If the mother had yelled out "$500 if you save my child" and someone does it just for the money, that is not necessarily a moral act, even if the consequences are the same. Thus Kant argues that for something to be considered a moral act it must be willed as a moral act.