Other people have given good answers, but here's another one: People's computers can already mount BigFAT-formatted drives.

Do you know what happens when you insert a btrfs-formatted SD card or USB stick into a Windows or macOS machine? It tells you that the drive is unreadable and asks if you want to initialize it. If the user answers yes to that question, the system formats the drive and all of their data is lost.

With a BigFAT-formatted drive, the system will mount it no problem, the user will be able to browse the contents, and the only weird part is that their largest files are split into parts.

Because you only need the software for >4gb files and block level access requires root or admin usually. This is can be fully userspace if your OS already supports Fat32 (it does).

1. BTRFS is a lot more complex.

2. Switching to BTRFS would be a breaking change. BigFAT wouldn't be. You can still use the card in devices that do not support it, without needing to reformat. Those devices would just lose access to some files.

Probably simplicity. It would be easier for a manufacturer to do a quick firmware update that implements BigFAT than having them support BTRFS.