Twisted doesn't just provide concurrency. Twisted also provides tools for building network servers and protocols. Haskell doesn't have anything unified; the community can't even decide which kind of I/O (lazy, iteratee/enumerator, handles) is best, let alone recommend a simple way to sit down and build fancy stateful networking.

Twisted, Event Machine, and Node (yes, even Node!) provide this. Haskell should have a library which does, too.