Why vote for a Democrat who supports Republican policies when you can just vote for a Republican?
I will go further and say that the parties are so polarized that trying to push to the center or into the other side of the spectrum could make you lose more supporters than win over independents. There were two big exceptions to this in 2006- Bill Ritter and Bob Casey Jr. but I think someone like Sestak or Salazar would have won by almost as much. People pretty much knew that giving W the green light for a third time in 2006 would be the same as giving a green light to having a party system in this country similar to Wyoming or Massachusetts, where only in a particular set of circumstances does any important voting goes on beyond the state's party's primaries. I think that's how the Republicans won in 2010.
