I think some of it is due to what Tim recommends. I thought I wanted Emeralds (not popular), he suggested Crawlers(relatively popular). I thought I wanted Abraxas (not popular), he suggested Mule (very popular). I have a feeling that the less popular models are mostly aimed towards a certain sound, more so than the more popular models, so Tim only reccomends those pickups to the costumers seeking that particular sound. This is generalisation of course, but the most popular pickups are generally agreed to be very versatile (Mule, Nailbomb, CS neck, etc). As opposed to the Riff Raff being strictly aimed at classic rock bite-y tone, Emeralds being typically 70s/southern rock, Painkiller a tight as spandex metal monster, Abraxas being too hot for vintage players and not hot enough for metal players. Still a generalisation (as I'm sure every BKP will be versatile in the right hands), I admit, but I think it has a say in which pickups are recommended and therefore in which are sold most.
(can't comment on single coils)
