The majority opinion (the court's decision was unanimous, but four separate opinions were published) held that the courts may not review the impeachment and trial of a federal officer because the Constitution reserves that function to a coordinate political branch. Article I. Sec. 3 of the Constitution gave the Senate the "sole power to try all impeachments." Because of the word "sole" it is clear that the judicial branch was not to be included. Furthermore, because the word "try" was originally understood to include fact finding committees, there was a textually demonstrable commitment to give broad discretion to the Senate in impeachments.
Furthermore the
Framers believed that representatives of the people should try impeachments, and the Court was too small to justly try impeachments. Also, the judicial branch is "
checked" by impeachments so judicial involvement in impeachments might violate the doctrine of separation of powers.