Not sure if I have a cogent reply to this. I guess it's a matter of both demand and developers time.
Several years ago I asked Thomas Knoll to add support for the P25 back as a personal favour when it first came out. He did, and Adobe has supported Phase backs ever since. I know that Phase One also now makes sure that Adobe has sample files and backs so that they can they can provide support for new backs as they are released.
I simply don't know if Hasselblad, Leaf and Sinar do similarly, or even if they wish to. It's interesting to note though that Phase One, who are in fact completive with Adobe in publishing a commercial raw processing program, also seem to provide the most support to Adobe of any of the MF back makers.