[csense]

> it's the short-sighted self-interested voters

Not really. The fact that people are individuals who always make decisions at the margins means you can fall into a sort of network effect hysteresis trap. If you have Party X who gives you 20% of what you want and is polling at 49%, Party Y gives you 22% of what you want and is polling at 49%, and Party Z gives you 80% of what you want and is polling at 2%, if you believe the polls are accurate, you have a much larger expected gain from voting for Party Y instead of Party Z.

This is true even if the voter preferences are universal -- if e.g. everyone would get 15%-30% of what they want from X and Y, and 70-90% of what they want from Z.