As you increase the Nyquist frequency the insertion loss usually goes up stratospherically. With PCIe 3.0 we were really pushing the limits of a lot of PCB mate

As you increase the Nyquist frequency the insertion loss usually goes up stratospherically. With PCIe 3.0 we were really pushing the limits of a lot of PCB materials and link budgets were already at the edge. It required a lot of innovation to break through those limits and we also had little demand to do so. So the past decade we've basically been moving from redrivers to retimers, we've been moving up to better PCB materials with lower channel losses, we've been preparing for a PAM-4 migration in a future PCIe revision, and tons of other little changes. They all add up. Plus a lot of enterprise gear moving processing to GPUs and storage to NVMe which is putting insane amounts of data over PCIe compared to older system topologies. This means we have money willing to buy better I/O which then filters down to us mere mortals over time as volume goes up and prices come down. So we've now got a fair amount of technical headroom, a good way forward, and people with money willing to pay for it. All of which are basically conducive to rapid generational improvements in performance.