Getting monads into Lisp in a useful way would require the introduction of features that would certainly make it a different language. You may consider this a feature but you'd be arguing with a lot of people who consider it a bug. (I mean that sentence very seriously; no sarcasm, no joke, it is an accurate description of the situation, nor am I implying I would find the idea horrifying... it would simply be a quite significant change to the core type system.)

You can get some monad behavior in CL now. Bind is fairly easy to write in CL but unit is messy. And CL's lack of automatic currying doesn't help.