Full-frame is one size - 24x36mm - so it's easy to design a lens that can go on different mounts and be used on a multitude of cameras. Medium format has a plethora of formats - 44x33mm, 45x30mm, 56x36mm, 54x41mm and 56x41mm - and that's just counting the ones up to 645-size. Which likely means a multitude of different, incompatible lenses, each designed for a different mount on a different aspect ratio.
Very good point, IMHO Hassy, P1 and Pentax should talk with each other and come up with clearly agreed on naming for the two "MF" sizes sensor sizes that are likely to survive, namely 33x44 and 41x54.
Such a standardization would really help the market and potential third party lens manufacturers to invest back in the market.
Cheers,
Bernard