Lets not forget she is not unbeatable. She lost in a primary to an inexperienced Illinois Senator who spent more time campaigning for President than actually serving in the Senate.
Right now all the polls are based off of name recognition. If we use the polls right now to predict the 2016 Presidential election than we should have President John McCain right now since at this point in time for the 2008 election McCain was beating Obama. In fact he was beating Obama by 2 points in New York.
Hillary Clinton is being pushed by the Democrats because she is their only hope. They don't have a young charismatic teleprompter candidate like Obama in 2008. They have no one besides Hillary that can have a shot at winning the Presidency in 2016.