There are still a lot of devices in use that don't support NVMe, especially older portable devices. And there are plenty of boards with M.2 slots that only support SATA, not NVMe, over some or all of those slots. It's one of the areas that board manufacturers often use to cut corners and reduce costs.
Getting a separate PCIe card is easy enough for a desktop, even a low profile one, but that's not an option across the board.
That being said, SATA SSD production was already probably getting near being dramatically reduced because NVMe is so prevalent now.