He’s running for his life.
He knows if he testifies he has to explain the Midnight Swat raids of Cohen and Stone. Why they leaked to the Press for maximum Exposure. Explain why this was not an attempt to get Trump to act so they could charge with Obstruction.
He also knows based on his interactions with the OIG his whole case was a fraud to begin with AND his lead investigator Weissman knew it before he started working with Mueller.
While still an DOJ Employee he can be compelled to testify. Now that he is private citizen he can and will lawyer up.
IMHO of course.
One last point (#4 if you prefer). Congress has already petitioned for an investigation (that was Mueller) and that investigation has returned NO INDICTMENTS.
They’re stuck.