I don't know of any specific reasons, but I can surmise a few. I would hypothesize that the most likely reason is for cost. A machine that makes diapers is damn expensive, a machine that does adult pull-ups is probably even more so because they are a bit more rare. I imagine that there are far more diaper machines available for companies to use than there are adult pull-ups. A machine purchased for that use would be limited to the designs that are readily available and it is widely accepted that most of those are garbage. That would mean a complete custom setup to create your own product for a very limited market.
I would also guess that there are copyright issues to be taken into account. The shape and design of those products is probably very heavily protected. It would be financial suicide to have a company design and build a custom machine that prints recreations of a major product, that will then be seized in the following suit. But this is just a guess.
I am more interested in why adult pull-up design is the way it is though. A not so disinterested company that is aware of this demographic did design and create their very own adult pull-up, they even made two distinct versions. NorthShore is the only manufacturer that I can think of off the top of my head to do that in a very long time. However, while their product is a massive improvement over existing adult pull up designs, it still follows the same "granny pantie" style construction. It uses a LARGE area of elastic material and a smaller pad. I'm not sure why this is the accepted design for adult pull-ups, and why there hasn't been more innovation in cut, fit, and design. It seems to me that Goodnites are by far and away closer to that of underwear in construction and fit, not to mention far more effective a product. I would be interested in hearing for NorthShore why they went with the design the did, or how it was even approached from a build stand point. I am still guessing that the "Pull-Up" design its self is a heavily trademarked thing and that it's nearly impossible to make an adult pull-up that wouldn't infringe on that design.
