I'm with Tony; my best guess & understanding is that the 20-blade diaphragms of late would be an extreme pain in the ass, if not outright impossible to implement in modern auto-focus / auto-aperture lenses. With the advancement of technology, remember, so has increased things like FPS; when you have a camera *body* (and shutter mechanism) that touts 12 frames a second, during which the aperture blades need to open up and stop down 12 times per minute as well, there is huge demand being placed on the rigoursness of the blade.

This explains why even high-end (e.g., Canon L) lenses tend to be very limited in terms of the number of blades (e.g., 5). If cost was the number one reason, then we would expect that the high-end, premium line of lenses to have more blades.