I think canon is actualy having problems with the sensor readout and heat. They just cant process the full sensor in a decent speed and convert it to 4k. Thats why they stick to 1:1 pixel readout.
The 1DX II and 5D IV are offering excellent 4k quality with 1:1 pixel readout. But The full-hd image that uses the whole sensor and processes it down to 2mpixel is realy realy bad. We actualy switched to 2 gh4 for some videoproduction where we only need full-hd (but want better quality than the 1dx and 5d "hd"). The filesize wouldnt be feasable for these shootings, since we collect a lot of footage in a studio.
This is possible since AFAIK, they still develop and use their own sensors for premier products. However, they have had plenty of lead time to develop this capability. The fact that others have gotten this to work and Canon, the most profitable camera company, has not, tells us something about their priorities. In other words, they may not emphasize the R&D to do it because they are satisfied with what in their view are "good enough" video capabilities.