Is it that hard to optimize a lens for wide open use?
The more the light rays have to be bent, the bigger the problems; for example rays of different wavelengths refract differently.
The light rays have to be bent more, if
- the focal length is shorter,
- the entrance pupil diameter is larger,
- the curvature of the lens is stronger.
Accordingly, it's easier to design a lens with long focal length, large diameter lens (so that the curvature is less), using only the center of the lens (small aperture). Of course, if the lens is large, a larger aperture will be utilized, but it will be sharper close to the center.
Turn these around: short focal length, large aperture.