Read her book "Going Rogue" and get back to us.
(I could tell you the answer, but I want you to do the research and find out for yourself. As long as we treat political elections like popularity contests, we will continue to get more Obamas, Clintons and Carters (Obama lite) in the WH than real leaders with real ethics and convictions!
She had an abundance of charisma and intelligence, millions of devoted followers, and was riding on the wave of the future (the Tea Party). And she had a very good job.
No matter what her excuses, she did not have to resign. There was no gun to her head. She just up and quit.