> Does anyone know why they have selected this versus the easier Score Voting option?
Instant-runoff voting (aka ranked-choice voting) is the law in San Francisco. It was passed by the voters back in 2002.
By the way, systems like approval and score voting suffer from the problem of voters having to vote against their favorite candidate to support a "lesser of two evils."
Think of an approval election with Donald Trump, Hillary Clinton, and Elizabeth Warren (where Warren is your favorite and Clinton is the lesser of two evils). Do you "approve" of Clinton, which would help defeat Warren? Or do you "approve" only Warren, at the risk of getting Trump?