1. Where the constitution and a statute are in conflict, one or the other has to control the outcome. CJ Marshall said the Constitution had to prevail in such a case.
2. The constitution gives SCOTUS certain original jurisdiction. SCOTUS can only take an original dispute, if it is one of the categories enumerated in the constitution. This issue was not in one of the areas that the constitution provides SCOTUS with jurisdiction. Lacking jurisdiction, the court could not hear or decide the case.
Marbury v. Madison is about the LIMITED power of the Court.
But, as is their wont, subsequent and lower courts have bastardized what the case really stands for, in order to flip it on its head and make it stand for the proposition that the court is all powerful and the final say. Neither those propositions is true, either. SCOTUS is not all powerful, and it does not have the final say.