A $10,000 medium format digital camera—both imaging back and body—is a remarkable achievement when you consider that medium format backs often cost two or three times that amount. It's an achievement on par with the Nikon D1 undercutting Kodak's flagship, a heralding of a new guard. Nevertheless, the Pentax 645D, while a good camera, is not likely to garner the same response as the D1.
The short of the reason is that the medium-format market is mature. When Nikon muscled-in on Kodak, it was the second player in a rapidly changing market. Medium format digital photography was also developing at this time, and had several sensor makers (and several body makers). In the past few years, competition has already whittled these players down through mergers and strategy. The Pentax 645D does not have a removable back, which limits its upgradeablity in a market that expects it, especially when the sensor itself is the equivalent crop on a true 6x4.5 sensory that an APS-C sensor is to the full-frame of those studio Kodaks.