He brought nothing to the table. He was from a safe state in a region that had already begun to favor Democrats. He had a very liberal record as senator with little that could appeal to moderate or independent voters. His public persona was underwhelming and offputting, and his super-rich background made him easy fodder for ridicule.
Why was he considered the Establishment/default candidate to begin with? Why did he have such an easy time winning the primaries? (Compared to, say, Mitt Romney in 2012.)
The front-runners were anti-war populist Dean and moderate establishment Gephardt, but in the run-up to Iowa the two went very negative against each other which basically destroyed either's chances. Kerry became the new frontrunner after winning Iowa and used his insider status to quickly clinch the nomination against all the outsiders that were still in the race, like Edwards and Clark.
I don't recall him being very popular even among partisan Democrats; he was basically just the consensus "least bad option" still standing after Iowa.
