Remember, Dean was the guy who, in the year 3 B.B.O. (Before Barack Obama, also known as 2005), implemented a 50-state strategy at the DNC, meaning Democrats were finally going to start competing in states they hadn't competed in since like the James K. Polk administration.
As Adam Nagourney points out in the article I linked to above, I think this year represented a perfect storm for Democrats which helped make a 50- (or 30-, or -25) state strategy possible. Consider this:
- Lots of people everywhere, including independents and Republicans, were pissed off at George W. Bush and, by extension, the GOP.
- The long primary battle between Hillary Clinton and Obama meant that Obama, the eventual nominee, had spent time, resources and committed staff in basically all 50 states.
- Obama had a simple yet compelling message, along with a shitload of cash (that helps).
- Changes in demographics worked in the Democrats' favor.