Because it is not trivial to process the larger amounts of data per second. Processors need to be fast enough and be able to dissipate the heat.
I can see that, but the new GoPro does 4k.
Processing power depends a lot on the algorithm used. The GoPro HEAVILY compresses it's video.... that's why it fits hours of video onto a microSD card. Also, the goPro shoots 4K video at 15FPS..... that's the same data rate as 2K video at 60FPS. Also, the GoPro is a VIDEO camera..... hardware is optimized for shooting VIDEO, as opposed to a DSLR which is optimized for shooting stills.
I don't have it with me to check file sizes, but on Sunday I shot a 55 minute long video (one continuous recording) on a GoPro..... compare that to a Canon DSLR where you can reach the 4G file size limit in about 2 minutes....
With such a lossy algorithm, you skip on trying to conserve the smallest details, so you need a lot less cpu power.... if Canons provided poor quality, they could speed up too