That's because M.2 NVMe isn't yet popular for NAS use - it's more for local storage or USB caddies to repurpose old drives as external storage as people upgrade.
M.2 NVMe aren't hot swappable - hence why they haven't been adopted, as most NAS allow hot swapping to replace failed drives. Even Servers don't really use M.2 NVMe for anything - they've only recently started standardising on U.2/U.3 for 2.5" NVMe drives, and E1.S /E1.L for smaller form factor drives.
There's also the lack of bandwidth when connected via a network - even with 2.5Gb Ethernet (whose adoption is still far from common), can only manage ~300MB/s - which is slower than even the slowest SATA SSD these days, so the speed of NVMe would be wasted.