Too much material on the double to have both stacked rods and the connected housing to fit in the area of the fretboard near/underneath the nut. The tilt back introduces more material in that area to accomadate for it, but it simply isn't possible with the straight headstocks like Strat & Tele styled ones.
If you look at one of the images of the side profile cutaway, you can see a single rod in place in the straight shaft, and see how once you introduce a foreign object in there, how very little supportive material is left to stand up to the tension of the strings pulling from the tuners in that very critical area.
The only workaround to this would be to add some heft in material around that, which would drastically take the "replacement" neck into a design that no longer resembles the design of the neck that it is intended to replace.
Want head adjust on a straight head?
Gotta go single, that's just how it is.