The issues with building it all around an MXM/deskmini are:
1) what is the challenge then? Just drop a bunch of raspberry pis in there while you are at it. It is much more fun
to create something if there is an actual challenge to it.
2) cost increases immensly! MXM based systems are significantly more expensive. Most people with consoles buy them for their cheapness. If they upgrade to PC they most likely would not spend extra on an enthusiast form factor.
3) making a kit for mITX conversion is much more accessable, as then people can pick whatever parts they want, and will be cheaper. With Deskmini based platform, you are limited to only buying a deskmini, and restricted to Intel and Nvidia.