But if you wanted to buy an OSX machine with up to 1.5TB of memory, you only had a brief window between when the new Mac Pro was announced and the old Mac Pro was discontinued to snag one before that option went away forever. The M-series Mac Pro only ever supported 192GB.

I agree, it is weird, but there were certain workloads (like those needing large memory) that wouldn't run anywhere else. Remember that M1 Ultra, at that time, was limited to 128G of RAM. M2 Ultra brought it to 192G. Intel Mac Pros could take 1.5 TB.