There are number of reasons as you can imagine. Those lenses would be enormous (think 10lb lens + camera). It is very difficult to manufacture large lenses (wit

There are number of reasons as you can imagine. Those lenses would be enormous (think 10lb lens + camera). It is very difficult to manufacture large lenses (with high quality), requiring a complete change in the tooling. It would also be labor intensive and thus only a very limited quantity can be made. At f/1.8, the lens price would probably be you just directly paying the technicians salary plus overhead.