Don't be so quick to assume you are right.
Perhaps the Democratic voters simply saw Specter as essentially a Republican who switched parties, not out of conviction, but out of political expediency and a desire to hold onto his Senate seat. Sestak did remind them of Specter's support of Bush, and implied that Specter could not be trusted to remain a strong and loyal Democrat. That message may have hit home.
Sestak's win was not necessarily a repudiation of Washington's policies or a reflection of anger over those policies. But, since Obama, and much of the state Democratic party hierarchy, had endorsed Specter, it does show that Democratic voters in Pennsylvania are not marching blindly in step with decisions made by either the party machinery or the White House when it comes to selecting candidates.
And Sestak is an appealing candidate with strong attributes. At a time when many people do long for change, he offered a viable alternative to someone who had been entrenched in the Senate for 30 years. In November, he may well prove to be a much more formidable Democratic opponent than Specter would have been.