It went a bit further than her being the spouse of a diplomat. Her husband is an intelligence officer and Sky News have called her a former spy.
There was no way either the US or UK authorities were going to allow her to go to jail given the national security implications.
Her being a former spy was questionable and never been proven. Besides, 'former' is the key word, she may have been, bit wasn't one at the time of the crash. Plus she didn't have diplomatic immunity
“The U.K. Government’s position is that immunity, and therefore any question of waiver, is no longer relevant in Mrs. Sacoolas’s case, because she has returned home,” Mr. Raab said in the letter that was shared with The New York Times on Sunday.
“The U.S. have now informed us that they, too, consider that immunity is no longer pertinent,” Mr. Raab said.