I am, of course, speculating as I've got the same specs in front of me that you do.
However, some clues it's the sensor vs DSP:
- Some modes with identical compression (4K fine vs 4K) seem to have differing heat issues
- The Digic X doesn't appear to be a slouch, with a 4x+ improvement in processing power it doesn't have the feeling of cut rate silicon, which canon has ABSOLUTLEY done in the past. Digic's that couldn't handle anything other than MJPEG, for instance.
- As you mentioned, there are lots of cameras that do 8K—but anything with this sized sensor has active cooling.
- There is a relationship between heat generated and surface area. Additionally, It's much easier to remove the heat from a 45mp 1cm smartphone sensor than a 35mm sensor.
- Active cooling tends to be attached to the sensor, not the image processor
Maybe I'm super wrong (I'm a designer not an EE) and it's canon being cheap. If I'm on the right track though, canon would have had limited options:
- Provide a camera with fewer megapixels. Nonstarter for lots of photographers.
- Put active cooling on it. Nonstarter for lots of photographers.
- Personally, I'd love to see canon release a 5Dc with active cooling and a larger body to match. They could also license additional codecs that aren't demanded by the stills/hybrid crowd.