The M-series Mac Pro was always for companies which were going to rack them and use them in render farms. Normal people was never its intended market. It was more of an Xserve successor.
Apple would need to design a different CPU for the Mac Pro, and the limited market doesn’t make it feasible. Descending the M-series CPUs from the A-series limits what the designs can do.
There are rumors of a CPU split in the Apple lineup. iPhone, iPad, iMac, Mac Mini, MacBook get the A-series, and MacBook Pro, Mac Studio. Mac Pro get the M-series. That would make sense, and might give them some room to expand the “Pro” procs.
A-series would already be at a disadvantage due to being designed for iPhones and the design parameters that entails compared to the M-series.