Because she is not a quitter. Quitting means giving up. She, obviously, did not give up. She STRATEGICALLY resigned as Governor.
I was nonetheless concerned when I read her lawyer say the Letterman incident was a factor. If she cannot stand up to him, how can she represent us as POTUS against Putin, Ahmadinejad, Chavez, etc.? To me a POTUS needs to have thicker skin, and be above those sorts of petty tricks. She fell for it.
To me the best response, if asked, would have been, “Lots of stupid people say lots of stupid things, and if I spent all my time responding to them all I’d have no time to run the state”.