Not precisely sure wrt PCIe 4.0, but I'd speculate that bandwidth limits of copper on FR4 is surely one contributing factor, driven by a cost-sensitive consumer

Not precisely sure wrt PCIe 4.0, but I'd speculate that bandwidth limits of copper on FR4 is surely one contributing factor, driven by a cost-sensitive consumer market. Cost-optimized interconnect solution may also be another. Somewhat related, I've noticed comm tech tends to follow a fairly consistent evolution: new enabling material/process, improvements to interconnect, algorithm optimizations, repeat.