Why are approval voting and score voting such good voting methods? This certainly wasn't the expected result when Dr. Smith's voting simulation was first run, and a lot of work has gone into trying to make sense of the result.
I've Got a Theorem...
When anyone first starts to dig into the vagaries of electoral systems, they will quickly hit upon the Nobel-winning work of Kenneth Arrow and his impossibility theorem. But there was another very important--perhaps more important--theorem that came along just a few years later. It goes by the mouthful-of-a-name of The Gibbard-Satterwaite theorem. You do have to admit "Arrow" is a much catchier, and certainly shorter, name; so we'll call this "G-S" for short.
What G-S proves is that, under any electoral system using rank-order ballots, if there are at least three candidates, there will always be situations where a voter who knew how every other voter was voting, would be better off by voting strategically. Always. Even if you allow for equal-rank preferences (which some Condorcet methods can handle) you still run into this problem. So the conjecture is that no single-winner, rank-order-based method could ever possibly support three strong parties, since any candidate would eventually run headfirst into this problem and the perfectly-informed voter will have to choose between honesty and strategy.
But approval voting and score voting don't have this problem with a third candidate. It is still always in your best interest to rate your true-favorite highest, and your true-hated lowest, and doing so will never cause the election outcome to be worse than any other outcome you could achieve. In short, there's no incentive to vote strategically.
Now, there are a couple counter-arguments that people will bring up at this point. One of them is that, since G-S is about a "perfectly-informed" voter, they claim that this means approval (and score) only work this well if there is perfect polling for an election. Which is clearly impossible, so we should obviously use INSERT_FAVORITE_METHOD instead. The logic here is entirely unsound. First, even if G-S doesn't guarantee the effectiveness of ratings-based methods, it certainly does guarantee the ineffectiveness of all ranking-based methods; to stump for a known-bad over an unknown-but-potentially-good, seems blindingly counter-productive. Secondly, the fact that even the best-informed voters may have to strategize to avoid a bad outcome will tend to cause less-than-perfectly-omniscient voters to hedge their bets, and strategically go with the lesser of two evils out of fear of the greater evil.
But a more bizarre (or perhaps just more brazen) argument is that, since approval and score can't pass G-S with four or more candidates, then they are clearly insufficient. Which is an absolutely mindboggling argument, since even school children know that three is still more than two, regardless of the fact that three is less than infinity. Are these methods perfect? No, they aren't. Are they able to deliver an outcome that it is impossible for any ranking-based method to deliver? Yes, they are.
The fact that these methods have no perverse incentives in three-candidate elections is probably a large contributor to their improved performance in three-candidate elections, even if they still aren't perfect. And we know that all methods do better when everyone is honest, so having nothing to gain from being dishonest probably accounts for something. And perhaps this improved performance with three candidates somehow carries over and provide better results with four-or-more candidate races too, since performance drops at a noticeably slower rate than the rank-order methods do.
Perhaps knowing why something works isn't as important as knowing that it works; but being able to explain why may help convince some people who refuse to believe that.