My best guess would be because a criminal trial would involve questioning her about her movements that day. Something the intelligence community absolutely doesn’t want happeneinf

Given the amount of previously classified information already released on this case (foreign powers can now accurately know that the US uses RAF Croughton for SIGNIT, will use diplomats and their spouses as intelligence officers, previous employment and housing history) the US will want to prevent any further useful information coming out. Even aspects like, when did you arrive, how stressful was your day, when did you wake up, etc are all going to be useful for adversary spies to know.

She’s fled so as to not be cross examined - she’s not coming back. Look at Raab’s lacklustre response - he knows it would be risky to the intelligence community for her to be scrutinised judicially