I'm pretty sure the "crowding out of readers time" effect you describe is less than the entire slate. I don't think the honest conservatives would actually read everything on the SP ballot; the work mostly isn't very good. After one or two really awful stories, they might just give up on the slate, or just glance at the others. Partisans (yellow puppy Hugo voters?), on the other hand, would just vote the slate. My guess is that the partisans are the ones who swung the nominating ballot.