It as combination of two facts:
1. Apple decided to design a pro workstation with only one CPU slot.
2. Intel does not have actual competency to Xeon, so they keep it with minimal investment and old architectures.
Why is so?
1. Each Xeon has only 40 lanes of PCIe. So if apple wants hight performance flash they have to accommodate for the lanes they have. So it will be (2) 16 Lanes for the graphics cards, and 4 for the PCIe flash storage and 4 for all the rest including 3 thunderbolt controllers. This decision by Apple has other negative connotations like low clock rate of the 12 core option (it could be tow 3.5ghz 6 cores). In the other hand it keeps power and costs down and the next generation Xeon will have up to 18 cores.
2. Even some personal computer chips on the Intel line have 40 PCIe lanes, so, one will have expected that since they keep Xeons one or two generations bellow their best, they will improve server related things. Memory bandwidth and IO are key performance components of server and workstation performance.They do not improve either. Go to intel ARC and you will see that Xeon supports lots of memory but is "low speed" memory. Some costumer processors also support 64GB of 1866 memory per socket.
Since Apple is not changing the mac pro enclosure any time soon, what we really need is AMD to launch a competitive server platform (unlikely).