I am presuming the associated circuitry was finalised in advance of UHS because I don't see how intentionally leaving it off would benefit them. I could obviously be wrong, but I can not come up with a scenario where the program manager made a decision to go with a slow secondary slot when a better option was viable and cost effective. I understand the notion of crippling, but that seems like a silly way to do it. The second card is nice but not strictly necessary. If you want the fastest performance, pull the SD. The 5D3 demographic isn't going to buy a 1dx due to a slow secondary slot, nor is 1dx demographic going to buy a 5d3 instead had it a faster secondary slot. Ergo I have to assume a more practical reason, such as timing.
I've worked projects before that tried to implement SD card support (from scratch) and the amount and quality of documentation are bizarre at best... the official SD spec is 500+ pages of information, and yet there are various exceptions and trickery that one must play to get different cards to work. SD != SDHC != SDXC. My guess is Canon had to weigh the risk of it not working right (and potentially jeopardizing the reliability of non-UHS 1 cards).
In other news, my SanDisk Extreme 128GB SD card just fell apart by itself after copying photos from a European vacation to my computer. Three of the four edges delaminated and the write protection tab fell out! Initially SanDisk rejected my RMA request because the pictures of the card showed "physical damage" which they don't cover. After I insisted that the damage was the result of the card delaminating without mishandling it, they decided to honor their warranty and are swapping out the card. For those of you with the 5D3, definitely record your photos to both CF and SD!