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Because you'd still have a cable, which is the one thing you're trying to eliminate for the sake of all-weather reliability. I reckon Shimano will be first with a fully hydraulic cyclo-cross disc set up, simply because they have the space to stuff a master cylinder into the body of a Di2 lever now that the shifter guts have been taken out. Rich kids and Shimano sponsored Pro CXers only, then, since that will be a big premium over the already eye-wateringly expensive Di2.

You are right, but your overplaying the "cables are unreliable" card. Infact a lot of guys with jumpbikes ride with cable / disks because you don't have to worry about ripping out / bending fittings. To me its more about getting brakes that stop more effectively in a compact unit, much the same size a normal brake, plus you can retrofit it to any bike.