McCarthy himself:
https://en.wikipedia.org/wiki/M-expression
The project of defining M-expressions precisely and compiling them or at least translating them into S-expressions was neither finalized nor explicitly abandoned. It just receded into the indefinite future, and a new generation of programmers appeared who preferred internal notation to any FORTRAN-like or ALGOL-like notation that could be devised.
Do you have links to the evidence? Not trying to be snarky. Genuine question. I wish to see more research and data in this space.