They had nothing

The Washington Post tells how the Trump team avoiding an interview with the Mueller team.
Central to the Trump strategy — developed first by Cobb and Dowd and later carried out by Giuliani, Sekulow and the Raskins, as well as Flood, who from his White House perch represented the office of the presidency — was to cooperate fully with every request for documents and witnesses from Mueller, including Trump’s written answers to some questions.
Their goal: to satisfy Mueller’s hunt for information to the extent that the special counsel would not have legal standing to subpoena the president’s oral testimony.