My limited understanding from what I have read would lead me to believe that prism size is the culprit. I am sure you have seen many of the lclassic porros that do offer huge fields of view. Almost all of them have huge prism housings to, well, house the large prisms. The larger prisms are needed to avoid any type of incursion into the light path.
How far off am I on this one Henry?
Since roof prism models have narrower bodies it would be difficult to create one with an extra wide field of view that did not have extremely fat barrels to accomodate the larger prisms needed.