Winning the Whitehouse involves a few key components, Values, the GOP with their Anti abortion and Anti Marriage equality stance does not sit well with the masses. They preach Freedom, but reality its freedom as long as they approve.
The other and most important component is marketing, The Dems do a far better job of packaging and selling it. The GOP is not a progressive party, in fact they have a silly desire to go back to 1955.
Society has always evolved and moved forward, the Dems have shown a great ability to adapt, where the GOP has not. Its very much like the dinosaur, they couldnt adapt to changes in the world and as a result they died.