Why would they? I would hope because someone wants to.
I'm guessing that's a reference to the small market for M-mount stuff (is it that small?).
I do notice that many 3rd party lenses I've seen lately are offered in a very wide range of mounts, but almost never including M-mount. And, I suspect it has nothing to do with technical difficulty (these are usually manual focus lenses, etc.) or design limitations (size, registration, etc.).
Regardless, I do hope more ZM lenses appear.