Her resignation letter came before that request for an investigation. They just held it until after the Kavanaugh nomination.
The second I heard this news, the first thing I thought was "she wants to make some money", and that was before I knew anything about her family's financial situation. She can now write a book, make speeches, and rack up some money to give herself and her family comfort and financial security. It makes complete sense.
And, she's young enough to do that for a bit and then re-enter the fray. If Trump decides not to run, I think she'd easily be the GOP candidate most likely to give the Democrats nightmares. And if he does run, then she'd be in the top tier for 2024.
I think this analysis is spot on. By leaving now, her reputation is intact and she's leaving on good terms. There is little upside to her staying on as UN ambassador beyond this point. She can go make some money and be ready to spin up her campaign org when the time is right.