Says who? There was even a republican candidate last year but she dropped out pretty quick. Hilary Clinton was running against Obama for 2008 and then dropped out, leaving him the Democratic choice against McCain (since Bush already had his 2 terms).
Sarah Palin had a lot of people who wanted her to run for president this year, but she chose not to (can't remember the exact reason).
The problem is that too many people still prefer men to be in control.
we think we are a progressive country but so many other countries have already had a woman for president/prime minister. The answer is that not enough voters get behind a female candidate. They can't handle the idea of a woman being in charge.