Here's what I don't get. Is there any reason more Democrats aren't voting for Clark other than the fact that he jumped in late?

It does appear that Dean is getting more airtime in news spots than the rest of the Democrats put together. The conspiracy theorist in me would believe that the Powers-That-Be have decided that he is the best candidate (ie credible but beatable) and that allowing Clark to win the primary would be far too dangerous for the incumbent.
Seriously though, I haven't the faintest idea. If Gore hadn't given his endorsement to Dean then the race would still be more open. The Clintons' endorsements could have been useful to Clark except for that little cigar incident.