I answered you once on that. A bunch of reasons: a) you can't appeal to independents as easily; b) you identify yourself with a bankrupt institution; c) you preclude running as an independent later; d) you blur your message into that of an alien party; e) you deprive yourself of a platform in the general election, when more people are watching; f) some states don't have binding primaries, but tricky and/or stacked convention/caucus procedures.
A better question is, why didn't all the dudes screaming at Nader run something in the primaries. Where was the 'Democratic left'? Bill Bradley? Please.
mbs