They basically had to be threatened into complying with the law.
Via Daily Caller:
The Department of Justice will allow Congress to review highly classified requested documents relating to the FBI’s investigation into the Trump 2016 campaign in conjunction with the White House and other agencies, White House press secretary Sarah Huckabee Sanders said Monday.
“Based on the meeting with the President, the Department of Justice has asked the Inspector General to expand its current investigation to include any irregularities with the Federal Bureau of Investigation’s or the Department of Justice’s tactics concerning the Trump Campaign,” Sanders said.
“It was also agreed that White House Chief of Staff Kelly will immediately set up a meeting with the FBI, DOJ, and DNI together with Congressional Leaders to review highly classified and other information they have requested,” she continued.