Why do higher frame rates require lower resolution? I've noticed that when comparing video camera specifications, higher frame rates usually require a lowered resolution.  For example, the GoPro Hero4 Silver can get 15fps at 4k, 60fps at 1080p, 120fps at 720p, and 240fps at vga.  I would think that the resolution of the individual frames would be independent of the rate of capture.  Is this because of bitrate capabilities of internal cables, or an encoding limit on the internal circuitry?  Thanks for your help.
I am not an engineer, but I think you've answered your own question. 

This is a bit of an oversimplification, but if t is the time it takes to record, process and store a pixel from a frame consisting of p pixels, then 1/(t * p) is your maximum frame rate. So if you increase the framerate above the maximum you need to decrease either t or p. And since t is fixed you have to decrease p - the number of pixels.