Because he didn’t know.
And he was obviously nervous enough about his own conduct - not knowing whether it would be against the law or not, or held against him or not, or spun to be something it wasn’t.
I think at one point he came very close to dismissing Mueller and it was a good thing that he didn’t because that would have made it look worse.
No, he knew he did not collude...
So he knew nothing would be found.
And the record now proves he used no executive privilege and gave extraordinary cooperation to Mueller.