I don't see how that's the case at all. In general, Americans hate their two political parties. Run-off elections would be welcomed. But neither the Republicans nor the Democrats want them, so we'll never have them.
My hunch is that it's more accurate--of course, not contradictory--to say that generally speaking, Americans don't like politics much (certainly our rate of participation is much lower than in other Western democracies). Consequently: 1.) the institutions that already stand soak up almost all of the energy that would be available for the enormous organizational task of creating a viable third party; and 2.) the vast majority of citizens simply lack the interest to
imagine real alternatives. Ask people what they imagine when they hear the phrase "third party" and I'll bet 90% of them would say "Oh, you mean like 'fiscally conservative but socially liberal, like a Bloomberg type thing? Might be OK.' That or some version of free-market libertarianism, the premises of which don't withstand even grazing up against inspection.
