Being new, you can take this opinion with a grain of salt. But I honestly think there is a major contributing factor.
I've seen quite a few coaches that create "super conferences" in worlds and would rather keep the dynasty going @ the DIII level, than move up to DII. With that happening, it's making it harder for the new coaches to achieve enough success to move up to DII, from DIII, or they don't feel comfortable making the move because they aren't achieving enough success in their own eyes, at the DIII level.
I've seen a few coaches that have excellent DI teams in one world, but also have an amazing DIII team that they are locking down in another world. Personally, more power to you, because it's your hard earned money that you're spending to play this game the way you choose. But I do think that this has an effect.
I looked at a few of the top DIII teams (being coached by excellent coaches) and they are just embarassing teams by 30pts a night. It could definitely be demoralizing and a deterring factor.
Just a thought.