Comment by Delk

Sounds like the short answer is "because there was no standard for the variable set by MS-DOS from the get go".

The background is that the issue hadn't existed in CP/M because there hadn't been environment variables. Perhaps if the issue had already been seen in CP/M, the developers of MS-DOS might have defined a standard variable to avoid it. Maybe. Other than that it doesn't seem to have much to do with CP/M specifically.