This is because PCIe 6.0 has officially been released, though there is no hardware that supports it yet, while the 2019 MacPro is on PCI 3 which was released in 2010. It is pretty bad since PCIe 4 was released a 2 years before the MacPro, and PCIe 5.0 has been out since 2019.
I am still excited for the next MacPro, though I doubt it will have any user available PCIe slots anyway, though hopefully it’s hard drive attaches faster than PCI 3. Likely it’s only expansion will be Thunderbolt 4 (which it will hopefully have more than 2 buses and 4 ports).