Five reasons, on the theme of electability:
1. Edwards is a better speaker. James Carville says that Edwards gives the best stump speeches of any politician he's seen, including Clinton. Edwards was unstoppable in winning over Carolina jurors in his old career, and he'll win over voters in his new one.
2. Southern Democrats do far better in presidential elections than Northerners do, at least since the Civil Rights Act. I'm cringing as I anticipate the words "Massachusetts liberal".
3. A Southerner won't drag down our attempts to retake the House. An inspiring campaign by Edwards will bring out Democrats from the South, while a Kerry campaign would increase liberal-fearing GOP turnout.
4. Edwards doesn't have a multi-decade voting record for opponents to nitpick. The GOP will distort and nitpick Kerry's voting record to no end.
5. He'll look way more likable than Kerry when he wipes the table with Bush in debates. Not that Kerry wouldn't beat Bush on the facts, but only Edwards would look like a sweetheart while doing it. That's how the southern lawyer wins.
(I don't really have anything against John Kerry, and I'd like to see him be on the ticket with Edwards. I just think Edwards would be better for the Democrats.)