My guess is that their custom software defined networking infrastructure is the main holdup. There is little known about the SDN infrastructure behind AWS, but

My guess is that their custom software defined networking infrastructure is the main holdup. There is little known about the SDN infrastructure behind AWS, but I would be surprised if large portions are not custom designed hardware and a homebrew control plane. It is very slow to iterate on the hardware side at such a scale for cost reasons. The cost structure is very different from server hardware, where the increase in performance per watt and rack density of newer hardware essentially pays for itself at some point. But a 10Gbps Port in your top of rack switch is good enough for many many years and multiple server generations, until you make the leap to a higher port speed. If the chip inside your SDN switch has no understanding of IPv6, you could only tunnel such packets in software at substantial performance costs. So it takes ages until the whole physical network is using new enough gear.