One reason is that assessing aerodynamics is a complicated process and the results will vary considerably depending on the approach you take. Mad Fiber claims good aerodynamics for their wheels but they acknowledge that results are open to interpretation. This is from their site:
To summarize our findings:http://www.madfiber.com/aerodynamics-article?tcc=3
(1) At zero degrees yaw and 30mph wind from straight-ahead, most top wheels are very similar. As long as spokes are minimal in number and profiled, and rims are aero, the drag numbers are tightly bunched. We find drag forces in the 120 to 150 gram range for all the top wheels, those with deep rims, minimal spoke number, and no serious aerodynamic flaws.
(2) As yaw increases, the order shuffles. With toroidal shapes, for example, drag gradually decreases, out to 15º, then steadily rises. The broad shape pays this benefit. For Mad Fiber, drag remains steady from 0º to 8º, rises from 8º to 10º, and then begins to decrease. At 15º, where all others are still increasing, Mads continue to drop. By 20º, the decrease steepens and they drop below any other spoked wheels. And this trend continues out to 30º, where our testing stopped.
For example, in late 2009, we tested a number of well known wheels. At 22.5º yaw, one of the best known samples generated 26% more drag than Mad Fiber. At 25º, its drag was 50% greater. At 30º, this competitor's drag was 109% greater, over double. Granted, these are extreme yaws, but this is what was measured.
My guess is that the overall design concept of the wheels came first (i.e. the lightest way to construct the deep section and join the spokes to the rim and hub) and the aerodynamic assessment came as a secondary consideration. This is pure speculation of course, but it would explain why they came up with a rim section that goes against most of the current wisdom on rim aerodynamics.