The fact is that there are many users who are die hard Capture One users that would never accept Phocus (or a Fuji tethering plug in to Lightroom, etc). There are enough of them - apparently - to make it worthwhile for Phase One to value the restriction of Capture One compatibility in the way that they have. For a landscape shooter looking to edit files, there are reasonable alternatives to consider outside of Capture One. But for many, Capture One is not just an image editor, it represents a much larger part of their software workflow.
Hard to know how big a factor this actually is in the average MF camera purchase, but I can say anecdotally that's a big one for me. One of the reasons that I can't see replacing my P1 with either the X1D (which I now have) or the GFX (which I will have shortly) as my primary studio camera is that they don't work with Capture One. My entire workflow is built around Capture One, and they're doing smart things, like the Tangent panel work in Capture One 10, that are delightful and save me even more time, which deepens my dependence on their software. I'm sure other folks love Lightroom or Phocus or whatever tools they're using for their production work. I'm just stating a personal preference.
The other thing to consider here is that the MF digital camera market is tiny. A few thousand new cameras are sold each year, in contrast to the 11.2 million interchangeable lens cameras that CIPA forecasts will be sold in 2017. Even if the X1D and GFX double or triple the size of the market, and every single person who bought one of these two cameras also purchased Capture One, that would add less than 5% to Phase One's top line. If I were Phase One and trying to figure out how to allocate my precious resources (they had 178 employees in 2015) I wouldn't be betting on the market tripling, nor grabbing 100% RAW processor market share for X1D and GFX sales. That makes the potential upside of adding this support much smaller, with potential downside of cannibalizing hardware sales and the opportunity cost of not doing other things that could drive larger increases in revenue, like, e.g., trying to get more of the much larger DSLR and mirrorless professional raw processing market share, and investing in the next generation of medium format hardware, perhaps including their own mirrorless offering.
Despite the fact that this disappoints some folks who would like Capture One support for the X1D and GFX--and I am in this group--it doesn't seem like a particularly surprising decision. I wish it were different, but I certainly don't fault them for focusing on the things they think will help them grow their business.
