I honestly think it is an economic thing -- most of the people willing to pay for medium format digital are shooting commercial where the end product is usually close to a 4:3 ratio. In film, shooting in a square was not a big deal as it just required slightly more film so if people cropped down, it tended not to really matter. With digital, that extra bit of sensor is a substantial extra cost given the difficulty in making sensors (the bigger it is, the more likelihood of an error in the chip, and consequently the higher rejection rate and cost). If 80-90% of their intended market is going to crop off that extra bit of information anyway, it makes more economic sense to stick to the 4:3 ratio.
As for the people who like the square -- it is more often artists and amateurs (I don't mean this in an derogatory sense...in the sense of someone who loves photography for making pictures, not for taking photos for catalogs, of products etc). In that market, people are less likely to want to pay 20,000-30,000 dollars for a digital back. They cannot amortize the costs and it is viewed as a discretionary expenditure rather than a business move. Hasselblad probably figures that they catered well to that market with the CFV -- they gave them a square sensor that worked seemlessly with the V series cameras and lenses that they preferred using. At the same time, they kept it at under 10,000 dollars so that it was an easier sell to this type of photographer. I am sure if they could make a full frame square chip for a price they thought would be profitable, they would do it.
Anyway, if anyone does it, it is likely to be Sinar or Leaf -- they have the camera system that is best suited to it (in the Hy6/AFi), and Leaf especially has shown willingness to make non-standard sized sensors. I suspect that the future of a square back from them will be dictated by how well the AFi 10 does.
Edit: I was writing this while Graham wrote his post...we were thinking along very similar lines.
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