Most literary prizes are decided by a small panel of judges appointed for the year. Several of science fiction's most visible awards work the other way, with the outcome decided by whoever has bought a membership.
The award grew out of a community, not an institution
Science fiction organised itself around conventions before it had institutional prestige, and the conventions were run by readers rather than by publishers or academics.
An award created in that setting naturally belonged to the people attending. Voting rights followed membership because membership was the only structure available.
That origin has been preserved as the field grew, so the prize remains an expression of what a self-selecting readership admired that year rather than a verdict handed down by appointed authorities.
Membership voting changes what can win
A jury reads a defined submission list and argues towards a consensus, which tends to favour books that survive detailed defence in a room.
A large electorate cannot read everything, so voters nominate what they have already encountered and enjoyed. Visibility and enthusiasm matter more than they would in front of a panel.
The practical effect is that popular, widely read work competes on more equal terms with difficult work, and a book with a devoted following can win despite dividing critics sharply.
Nomination is the decisive stage
The shortlist is produced by the same electorate, and nominations are usually far more thinly spread than final votes. A relatively small number of nominations can place a title on the ballot.
This makes the nominating stage the point of greatest leverage, and it is where organised campaigning has historically had the most effect on outcomes.
Award administrators respond by adjusting counting rules, since the mechanism used to aggregate preferences determines how much a coordinated bloc can achieve.
Juried prizes exist alongside them
The genre also runs juried awards, and the two kinds regularly select different books in the same year.
That divergence is informative rather than embarrassing. It marks the distance between what a committed readership loved and what a panel judged most accomplished.
Readers who use awards as a shopping list are usually better served by knowing which type they are looking at than by treating all prize stickers as equivalent.
What the model costs and returns
Reader voting is vulnerable to campaigning and reflects the composition of the voting body rather than the field as a whole.
In exchange it produces a result nobody can dismiss as the taste of five people in a room, which is the reason the genre has kept it.