- they think Obama is cool and all the cool people are with him (Jimmy Carter, college students that don't know electoral college from community college)
- they realize Obama is gonna win and it's better to hitch your trailer to a winner (e.g. John Edwards)
To me this is how I see it: Obama is a risky choice since he can't win the "blue" states. You can preach all you want about a single America but to win elections in this country: you've gotta carry the states of your party, steal some from the other guy's party and hope the independents and old people don't confuse you for Nader. So fair or not, hopeful or not, there's a clear path that each party's candidate must take to get the required number of electoral college votes (of course it helps if your father has pals in the supreme court that can swing things your way. that's the ultimate joker.)
Clinton has demonstrated that she can win using the tried and true democratic path (after all she won the big "blue" states).
Obama is saying to hell with all that tried and true formula. I am writing my own rules. I am going to contest in every state and run a true national campaign. None of that blue state, red state for me.
So the question for you, as a voter, is are you willing to take a chance with Obama? Or will you go for the tried and true formula?
Of course, with Obama racking up so many super delegate endorsements you may not really have a choice!