Ice Lake SP Xeon chips offer significantly more cores than the previous Cascade Lake chips (up to 40 cores/80 threads, vs. 28/48 in Cascade Lake) and early reviews show them to be about 50% faster single-core performance then the prior generation. The people who buy the Mac Pro (namely big movie production companies) want Xeon chips even though i9 chips are faster for many tasks because they want things like ECC memory, additional PCI-E lanes, support for massive amount of RAM, expandability and repairability.
Adding these features is a very real challenge for Apple to pull off with the M-series SoC design. It makes sense that they’ll introduce an M-series model with more limited capabilities but blazing speed for a broader subset of professionals (graphic designers, photographers, videographers, basically anyone who makes a living in Adobe apps) while maintaining the Intel Pro for a few years for the niche subset of users who really need Xeon-grade power and reliability.