Because among them were the Crossfire-Hurricane docs, which implicate a lot of Stasi scumbags in felonious activity.He never hid them. He showed them to the FBI on multiple occasions. His claim was that he had a right to keep copies of documents after his presidency, as every president before him.
Even vice president and secretaries of state have done that. Hard to see a valid reason why this no keeping classified documents rule that never applied to senior officials before suddenly was applied to Trump.