I am not a lens developer, but as far as I can make out it appears to be a direct result of lens specifications. A lens optimized for large apertures will not necessarily offer very small ones. Your 50mm lens is a prime example of this. Then again some lenses offer extremely small apertures, but I doubt that those lenses have been designed for minimized apertures. It is just possible without much ado to deliver extremely small apertures with some lens designs (for example tele lenses) and it is more difficult for other lens designs with shorter focal lengths (for example standard range zooms like the 24-70mm). There are some articles on CanonRumours about lens design, which will shed more light on this topic compared to the tiny bits of information I can provide.