There are some other potential reasons: - Simpler PCB routing (= fewer layers = cheaper board) only using lanes from the CPU closer to the slots - Slots still work when 2nd CPU is not installed without extra chips - Board is already not designed around heavy I/O use cases given lack of PCIe 4.0 support

This definitely feels like a board that's mostly intended for compute-intensive workloads that do not need much PCIe bandwidth.