Different designs for different requirements. Small format users differ from large format users in a few key requirements. First, small format cameras have a fixed relationship between the lens plane and the film plane. This changes lens designs markedly (especially when you have to clear a swinging mirror).
Originally Posted by Shaggysk8
Second, most small format users are handholding. This implies the need for speed, which in turn implies the need to shoot wide open and at apertures near wide open. Most LFers are bound to their tripods and tend to shoot at apertures in the f/16 and smaller range. This requirement changes lens designs too, in that the designer can't count on smaller apertures to minimize aberrations, and has to design the lens to be sharper wide open and have better off-axis light fall-off characteristics.
Third, LF use requires much larger image circles to allow for view camera movements.
What you end up with is that LF lenses tend to be simpler, and tend to be closer to symmetric designs. Small format lenses tend to be asymmetric and considerably more complicated.