The main reason is the auto-aperture function, where you can meter and focus wide open, and then stop down to the shooting aperture automatically as the mirror flips up, before the shutter opens. Starting with some late Graflex reflex cameras, auto-aperture is a relatively modern phenomenon, and it's hard to make it work with 20 aperture blades, without adding an uncomfortable amount of shutter lag. My Zeiss 85/1.4 ZE has 9 blades, which is about as many as you'll find in a small SLR lens with auto-aperture function.