In part he cannot fulfill many of his campaign promises because most of them are extreme and could be deemed to be unconstitutional and be reversed, that is what happened when he signed an order to ban people coming to the United States from Muslin majority countries, someone took the case to the courts and a federal judge ruled out the ban on people because of their place of origin and their religion from entering the USA was against the constitution.
That is how it works in the United States, the president is one of the maximum authorities but the power in the country is divided in several branches, that prevents from one single person to govern at their will and whim, making the rules of the country and having nobody to challenge whether those rules/executive orders are okey or not.
That is part of why many within the political left are scared of Trump co.ing back to power and try to turn the UsA into a dictatorship, getting rid of the separation of power, so there will be no judge to challenge his executive orders. If the Senate and the house of representatives are also under the control of the Republican party after November, then it would make things easier for whatever plan Trump has on mind.