Part I: The Set Up
Imagine the voting populace as people out sunning themselves (or swimming, watching whales, searching for buried treasure, building sand castles, etc.) at a beach. Along the beach there runs a long boardwalk. On the boardwalk are two ice cream vendors, one "Republican" and one "Democrat." Both have the same ice cream for sale (exercise of political power), but the selling point of the two vendors is convenience.
Initially, the vendors try to position themselves in the way that best suits the average beachgoer. The Republican cart is about one third of the way from the right end of the beach and the Democrate cart is about one third of the way from the left hand side of the beach.
This works fine for a while, as both of the vendors get about the same number of customers, and everyone is happy to have a convenient source of ice cream.
One day, however, the Republican vendor realized something: if he moves a little to the left, he'll be more convenient for some of the folks in the "middle third" of the beach, who would otherwise go to the "Democrat" cart. The "Democrat" vendor realizes the opposite thing (he needs to move to the right) after sales start to drop off. Eventually, we wind up with two carts essentially in the very middle of the boardwalk. In reality, there are some aspects of tradition, inertia, marketing, and so forth that keep each of the two vendors from being right in the very middle.
Part II: The Movement
There is no reason that Republican vendor is going to move much to the right. If he does, the Democrat will follow, and now more than half the beach will find the Democrat more convenient. The same principle holds true for the Democrat - if he moves to the left, the Republican will follow and more than half the beach will find the Republican more convenient.
Frankly, the people at the middle of the beach are pleased as can be about the arrangement. They have easy access to the ice cream, regardless of which vendor is slightly closer to them. On the other hand, those at the ends of the beach (the "extremists" we will call them) are pretty unhappy about the "compromised" position of the vendors. It's surely not convenient for the far left to go to the Republican cart, but it is only slightly more convenient for the far left to go to the Democrat cart.
Thus, both ends of the spectrum would like to see either a different system (a three-vendor system) or at least a change in the position of the vendors carts. The former approach turns out to be untenable in practice on this particular boardwalk. Although it sounds good on paper, for some reason it just doesn't work in practice.
With respect to the latter approach: there are two main ways the extremists can change things:
1) By attracting more beachgoers to their cause. This, in effect, rebalances the beach. It gives the two vendors a reason to move slightly in their direction.
2) By leaving the beach. If they leave the beach, the vendors will move slightly away from the direction that they were.
Choosing not to vote in a given election is leaving the beach. The result when "Conservatives" refuse to vote for the more "right" candidate is that the vendors both move to the left. Likewise the result when "Liberals" refuse to vote for the more "left" candidate is that the vendors both move to the right.
Just a little ice cream for thought.