Although there are many backs for the Hasselblad all of them are smaller than the film. Why don't they make it the same size. I don't ask for more MP just the same size as the film so that when you switch from the film back to the digital back your lens still works the same.
The larger the sensor the higher the cost.
First, you obviously get less devices out of a single silicon waffer. So each device costs more, given a fixed cost per waffer.
2nd, in general, larger devices have a lower yield rate, because of defects. This relates to the first item.
Numbers that follow is for illustration only, not actual production numbers, because I have no idea what the cost of a single waffer is, nor the current yield statistics.
If you can make 100 DX size senors out of one waffer, you may be able make only 40 FX size sensors. The waffer size is constant, so the larger the sensor, the less sensors can be made out of the waffer. Divide cost of $5,000 per waffer by the number of sensors per waffer. The DX sensor would be $50, and the FX sensor $125. The larger sensors will cost more per sensor.
Now add failure rate. So if you have a 30% failure rate, you get 70 DX sensors @ $71, but only 28 FX sensors @ $179. The unit cost goes up because you have to divide the cost of the waffer by a smaller number of good sensors.
In actuality it is harder to get a GOOD large IC than a smaller IC. Because the larger IC has X times more area than the smaller IC, it is X times more likely to have a failure. At least this is how it was explained to me by an IC manufacturing engineer.
You can run the exercise for any size sensor, and do the math with the same results, the larger the sensor, the higher the cost of each sensor.
I would love to have a full 6x6 format sensor for my Hasselblad, but I'm not going to hold my breath waiting for it, nor pay the $$$$$$ for one.
Similarly the large format guys would love a 4x5 sensor, but you would only get ONE sensor out of a 7" waffer. Any defect in that sensor and it is gone. The production yield would be terrible.