Why no support for Sigma RAW? Or even DNG RAW?

The Foveon technology is quite different from the Bayer Filter sensors used by most digital cameras. As far as I can tell, most cameras supported by Apple's RAW support use Bayer Filter sensors. It may be one of the reasons, why there is not yet support for the Sigma cameras.

Not all DNG formats are supported by Apple. Previous versions of the RAW Support document stated "DNG files must be generated by the Adobe DNG Converter with the “Convert to Linear Image” option turned off."

Only Apple and Sigma know the reason. As Apple is not here, you are just asking fellow non-Apple users to speculate which the Apple hosts discourage. You can contact Sigma, or send feedback to Apple, and the Photos Project Team, or macOS Project team.

Only users here and no one know why anything (and speculation on Apple policies is not allowed anyway) - we do know what is supported - RAW support is specific to each camera, there is no general RAW support - these are the currently supported cameras - Digital camera RAW formats supported by iOS 11 and macOS High Sierra - Apple Support

One presumes that Apple supports cameras based on the popularity and I've never hears of a Sigma Camera nor have I ever seen one mentioned here and in looking at their web site it looks like they have an extremely limited offering of DLSRs so that may be the reason Apple has not bothered to support them

LN