Top canon and nikon SLR's don't have integrated flash - and entry level SLR's have. I think philosophy is that when you have a professional body - you need good external flash. Most of medium format cameras are considered professional.
holgas are aimed at the amateur market for people who just want a picture of some sort. Other medium format cameras are aimed at folks who hate pictures that look like flash pictures unless the really want that look.
The modular aspect of most medium format cameras also encourages the use of detachable flashes, versus the "do-it-all" nature of a lot of mid-range and entry level 35mm SLRs.
I don't think I've ever seen anyone that owns an H camera actually use that thing as an onboard flash, although I suppose it could serve as a optical trigger for other strobes.