I am not a CMYK user, so this is just an assumption. The tools are RGB related and not CMYK. In example I can setup a softproof view of the image and see the colors how it would look like in CMYK. Then select the brush tool, choose color blue 0000ff and when painting it looks nothing alike, because it is interpreted as a different color to view; much more muted and almost purple. But its still blue in the background and all filters and tools will interpret it as RGB blue when making calculations. The channels of the image is still RGB instead CMYK. There is always this guesswork and interpretation involved, which would probably be not there if there was full native CMYK support.

Edit: Tools and filters in example have sliders for RGB colors, but not CMYK. You still need to think in RGB and you have to think how it converts to CMYK at the same time. Not ideal for people who are used to native CMYK editing in other tools.