If you by "left him there" mean that AS left Harry Dunn on the road where the accident happened, that is not correct, as the police interviewed her at the scene of the accident, and also the day after when they informed her about his death. AS told the police then that she was planning to stay in the UK, but about a week later she, and her family, was back in the US.
The question is, was the return to the US something AS and/or her husband requested, or were they ordered back to the US by their employer/s? It seems that both AS and her husband had jobs that probably were security classed, and their employers might not have wanted them exposed, but if that was a reason, it backfired. Whether or not AS might have been willing to go back to the UK for a trial, the whole issue had became too politically sensitive to allow her doing that, as that would have been seen as an admission getting her back to the US had been a wrong move.