Because haskell has such a library (a few actually) and ocaml does not. So in ocaml you either have to give up safety, or write a bunch of verbose boilerplate.

Because haskell has such a library (a few actually) and ocaml does not. So in ocaml you either have to give up safety, or write a bunch of verbose boilerplate. There's no reason someone couldn't write a lens library for ocaml of course, but the much smaller community makes for far fewer libraries.