probably a combination of licensing fees, USB interface costs, power supply needs and 3.2 ext drives/cases not being all that widely used yet made Sony decide it wasn't worth it (yet?) - btw the 2 USB-A ports in the back are also 10Gbps
the XSX only seems to have 5Gbps USB ports and per DFs internal vs external comparison the difference between loading X1 games from internal (2.5GB/s) and an ext SATA (500MB/s) is pretty miniscule as the games haven't been coded to make full use of the speed, unlike native XSX/PS5 games
Wow I overestimated both the Series X and PS5's USB speeds I thought the USB-C on the PS5 would be 20Gbps and that all 3 of the Series X ports were just 10Gbps. 5Gbps covers the standard SSD speeds of the Series X and hopefully we will get some info on the same tests done on the PS5 soon to see if the 10Gbps speeds make much different on BC games on there.
It's going to be interesting to see what the next gen consoles do for SSD prices as NVME drives at 1500-2000MB/s aren't much more than the 500MB/s SATA SSD's.