It's not even tribalism, the fuckers were cut from the same cloth as Joe Lieberman, where they never saw a policy that fucked their constituents that they didn't love. It was proven repeatedly back then, but they weren't winning because they were the best choice, they were winning because the DNC was shorting basically everyone else to keep them afloat. It was the reason they objected so hard to any attempt to change focus (and hated Dean in particular); the Blue Dogs were money sinks, had to be appeased to get any legislation done (and were basically Neocons with a different social bend). They were the worst sort of triangulating shitheels and even by the staggering standards of Congress, they were fucking awful.
I retain that the best example of why the Blue Dogs were so hated was the DNC's mad drive to keep Lieberman in office. For the uninitiated, he was such an unmitigated piece of shit that his home district had enough of his shit, and, in a surprising move, successfully primaried him - and won. At this point, the fucker ran as an Independent, now with the DNC flushing basically infinite money into the race on his behalf because we can't possibly lose the establishment's favorite greasy bastard.