Because it has.
Heck, people are talking about Mitt Romney trying again in 2016
Well FWIW I think Romney's shot has come and gone as well.
So why is Sarah Palin different than any other politician, who lost an election but seeks to try again????
Because Palin has about a dozen other people running for President who will rip her to shreds in the primaries. And most of whom are far better choices than she is.
good, im sure you can show us those others and how they are superior to her and her stance on the issues.