For what it's worth, PyPy had a multimethod implementation that it used to power some of the more polymorphic builtin functions in Python. They recently started killing them off as too difficult to reason about and maintain.
# You can only dispatch on the class of all arguments. # You can't prefer one method over another. # You've got to use the Python class hierarchy.
I suspect the last one is the real killer: you can't detangle yourself from Python classes, and it doesn't interact well with the Python class system. Idiomatic Clojure code, on the other hand, treats classes as a necessary evil in extremely limited circumstances and allows you to construct arbitrary hierarchies.
All of which is insight that helps explain to me why the Python implementation looks so limited to me. Language design is hard, especially when you're popular.