Why did a Ph.D., an academic, and a computer scientist not know about UNIX in 1974 or so? 1976? In 1976, some (many?) universities had source code.
Some knowns/givens at the time ...
1.) He was a language/compiler type person -- he had created PL/M and that was really what he was originally trying to show off. As I understand it and has been reported in other interviews, originally CP/M was an attempt to show off what you could do with PL/M.
2.) The 8080/Z80 S-100 style machines we quite limited, they had very little memory, no MMU, and extremely limited storage in the 8" floppies
3.) He was familiar with RT/11 and DOS-11, many Universities had it on smaller PDP-11s as they ran on an 11/20 without an MMU also with limited memory, and often used simple (primarily tape) storage (DECtape and Cassette's) as the default 'laboratory' system, replacing the earlier PDP-8 for the same job which primarily ran DOS-8 in those settings.
4.) Fifth and Sixth Edition of Unix was $150 for university but to run it, it took a larger at least 11/40 or 45, with a minimum of 64Kbytes to boot and really need the full 256Kbytes to run acceptably and the cost of a 2.5M byte RK05 disk was much greater per byte than tape -- thus the base system it took to run it was at least $60K (in 1975 dollars) and typically cost about two to four times that in practice. Remember the cost of acquisition of the HW dominated many (most) choices.
I'll take a guess, but it is only that. I suspect he saw the S-100 system as closer to a PDP-11/20 'lab' system than as a small timesharing machine. He set out with CP/M to duplication the functionality from RT/11. He even the naming of the commands was the same as what DEC used (e.g. PIP) and used the basic DEC style command syntax and parsing rules.
Some of us, know how to find him. I know that at least at one time, was made aware of this mailing list and have been invited to join it. It is his choice to not be a part.