Well, according to one source:

At X, her base salary was reportedly $6 million, according to Market Realist. In addition to this, she was eligible for up to $2 million in annual performance bonuses and received $4 million in stock options spread across a three-year term.

So if she stuck around another year she'd have gotten another ~$1.3 million in stock options. Guess she figured it wasn't worth it.