I agree with giantguy99 that production costs is probably the biggest factor. Most medical diapers are made on a much larger scale and bought more often when compared to AB "designer" diapers. And more often than not they're bought in bigger bulks (think a hospital buying them for all their patients) and paid for by medical insurance companies so companies that sell medical diapers have even more incentive to cut costs which is why many (but not all) medical diapers are starting to become far less superior in performance when compared to AB diapers. As oppose to us here that are willing to shell out the extra dinero to get the best of the best, lol.
Dogboy and Bambinod bring up other points too that I think also attribute the decrease in plastic backed diapers, but I think the costs per unit are the main factor.