Because it is only $45.95 more than a replacement body cap (with the added bonus of being the same size, with a lens built into it).
There have been a couple of threads on this already. There are reasons that are very good ones for some people and totally useless for others. I quite like it but its not for everyone.
First you have to understand this isn't a lens that is also a body cap, it is a body cap that is also a lens. You need to think of it that way, and should not be comparing it with lenses. It serves a different primary purpose than they do.