I have a number of different ideas going through my head (possible licensing, low volume of part availability, power requirements) but haven't found anything to demonstrate what the actual problem is.
I've had a computer with USB 3.0 ports since 2008 but I have had zero devices that use it in all that time.
I do agree that USB 3 would help many kinds of shooting, and there's no problem with transfer speeds to a PC - if hard drive speeds are a problem then you need to set up RAID or maybe even use a SSD.
It would have been nice to roll out with the new cameras, but not essential. I think Canon is right to play it safe on the CF front too, although Nikon is moving quickly to the new QXD or whatever it's called, many users are not ready to move to a new format, or to deal with the smaller support base available for the new format.