The issue is that a DNG filewrapper adds nothing positive to a Capture One Raw conversion, only adds more things that can go wrong in the conversion between color models and creates a need for more provisions and validity checks (while the quality of the conversion is not better).
In my view, Phase One has a little too great a focus on Capture One Pro as a RAW converter, and a RAW converter foremost. I understand it from a marketing perspective - this is the niche they want to dominate. Their RAW conversions, and adjustments, are arguably the best in the business.
But some users of Capture One Pro want more than the bare bones RAW converter Capture One Pro once was. They see it as time for it to expand into the DAM that Lightroom is and Aperture was. And it is moving that way - a catalog, keywords, metadata presets, and the like, are being added.
But they've been slow in coming and buggy in implementation. And, as a DAM, CO needs to be more accepting of file formats other than plain manufacturer's RAW files. DNGs, PSDs, TIFFs (all flavors), and so on, are first-class image citizens for many.
I wish Phase One luck with this because I, for one, would love to move from Lightroom to it. But the missing, or poorly implemented, DAM features trip me up each time I give it a serious try.