Making convincing animation is also harder than it looks. It's an entirely different skillset. 3D animators at the big studios (Disney, Pixar, Dreamworks, etc.) are required to be able to execute 2D animation and then combine it with their 3D abilities. 3DX does have the luxury of having more emphasis put on model features and details (pretty skin and big tits) and less about movement (sucking and fucking are pretty simple, looped actions). You'll notice that 3DX animators bank much more on looped animations and provide variety through camera movements. Any "story" part of an Umemaro work usually looks much stiffer and robotic than the sex scenes.
I think lack of collaboration also results in lack of content. The more complex a project, the more it benefits from multiple people on the same page. Porn content production is a solo operation most of the time., which means more projects that languish in the development phase for various reasons.