Because it forces people to choose between stupid, ineffective, "lesser evil" support of a Democrat and stupid, ineffective symbolic support of "independent radical political action." What substantive political difference did either of Nader's previous two presidential runs make? None, as far as I can see - he just provided another flavor of Coke for voters to consume (the fact that he was a nicer flavor than Bush, Gore or Kerry is irrelevant). A meaningless spectacle with three options is no better than a meaningless spectacle with two options. Nader props up illusions in bourgeois democracy in precisely the same way that the left wing of the Democratic party does.
Now, running a third-party presidential candidate might in principle be an element in a genuine political mobilization, but the point is that it's the mobilization which is important, not the presidential run. I'm not aware of any of the currently-existing third-party candidates in which the presidential run is in fact a part of such a mobilization, though maybe I'm missing something. I do find it hard to imagine in the current political context a campaign for which running a presidential candidate wouldn't just be a waste of resources; I'm not sure of any current campaign that has enough national momentum and organization. Again, though, I might be missing something. --
"Why must man's vocation always be to distinguish
himself from animals?" http://blog.voyou.org/ -- Baudrillard