The big difficulty in adopting it in any current codebase is that most current pieces of software are not written in haskell. The huge advantage of projects in C like OpenSSL is that you can produce platform-native shared libraries that can be used easily from any other language.

A bit of research shows that it's possible to make shared libraries with haskell, but not quite as straightforward to use them. I have no practical experience about how troublesome it would actually be.