Thanks for this. Do you means Mux as in "mucks" or Mux as in "Mc's (like in McSweeney, McDonald and so on)"?
The problem is that Mx, like Ms, violates English phonotactic principles. The language abhors a schwa (the upside down e - /ə/) in a one syllable word because it cannot carry word stress, which is important in English. If you think about it, the words "the" and "a" which are pronounced one way when unstressed are pronounced completely differently when stressed. A lot of the weirdness comes from this, I think.
Another reason is that Mr and Mrs are abbreviations, so there is something actually to pronounce. This is not the case with Ms and Mx.
I use Ms much, much more in writing (it's my default if my correspondent has not expressed a preference) and on those very rare occasions I actually have to say it, I tend to use "Mizz" to distinguish it from "Miss".
Edited to give clearer example