Sure there may be pockets going more conservative, but overall people are becoming more liberal and "moderation" is becoming what many people want.
Actually, a good number of those Kerry votes were "anyone but Bush votes" without any other outlet. At least in 2000, the tie-died-tree-hugging-latte-swilling-faux-disaffected crowdn had Nader to pull the lever for. Not so lucky this time around. These were simply Howard Dean votes (and Howard Dean money, which is why he's still on the scene. The dems learned they can both keep Dean's money machine and trick his legions into believing they've been heard, by keeping Dean himself far away from the ballot, but giving him a sinecure with prestige).
Interesting thought experiment; suppose Dean managed to survive on some third-party ticket. Now run the numbers in a race of Bush-Kerry-Dean. Does John Kerry still garner 56 million or so votes? Does he still carry any major electoral jackpot other than California? Answer: Hell no. I believe he loses Washington, Oregon and possibly Wisconsin in this scenario.
Don't believe for a second that Kerry was all that popular amongst democrats. The early hype for Dean and the screams for "Hilary NOW!", should have made that apparent. However, he was the candidate the apparatchiks wanted and so democrats did what they were told and voted for him.
Had there been a viable, credible third-party candidate, even serving as a symbolic protest (like Nader in 2000, who cost Gore Florida), Kerry would have lost by a much wider margin.