I believe this is because both Flex and Stax support NFC, as the backup method used by Ledger requires such communication channel. I’m sure if their other products supported NFC, they would also extend this backup to those devices.
That's true for the current range of available Ledger hardware wallets. But if the company wanted to, they could have developed Ledger Recovery key to be compatible with hardware wallets from other brands as well. I am not sure how many such devices that are equipped with NFC chips are out there. I think Cypherock is one of them and the Tangem Ring. I think OneKey communicates through NFC also.
The point I was trying to make in my previous post is that there is a purpose behind their decision to only support Ledger hardware. They want you to buy their devices if you want to use Ledger Recovery Key and vice versa.