In my not so professional opinion, it could be having to make them fit every situation from those who bed ridden, different positions of sleep, to just over the years it has been discovered they needed to be that way. They make them generic to fit all those situations instead of making a different diaper to fit every situation. This probably keeps cost down for the company and to the consumer. For an example, I use the front, middle and some of the back portion(if I can't change and have two episodes) the most during the day. Then at night mostly the middle and back portion (till I get out of bed than my bladder says time to go again. This is just a guess about them.
I know when I was in my learning curve, I folded the top part down and thanks to my anatomy the padding would soak the top part of my jeans. Talk about learning the hard way. Now I wear a tank top t-shirt to hide the top of the diaper.
In a strange example, think of a person walking into a car dealership and saying I need a car which has a stationary seat that fits a 68 inch person. So instead of a car manufacturer makes movable car seat to fit tall and short people of the world. If that comparison even makes sense.
