There's also the notion that most muzzleloaders really don't shoot all that much, and wouldn't want to buy bags of different sabots to fit all the bullets they might want to experiment with-- for those that really are interested in pushing the peformance level a bit. Plus, most people buy bullets pre-packed with MMP sabots.
As it is they are $7.25 for a bag of 50, including shipping, and a buck less than that if you order sabots at the same time. Not really the treasure of Sierra Madre for someone that wants to give them a test drive, and they fit all .50 cal. MMP sabots you might have. That covers PR Bullet, Winchester, Buffalo SSB, Hornady, Barnes, etc., to my knowledge.