Neither does.. well anyone. Dropbox decided to drop support for BTRFS and pretty much any other Linux file system that isn't ext4, and to my knowledge still hasn't explained their rationale to this day.
Sharing the reason is either a legal risk, or a loss of competitive advantage risk (some optimization that only works when their whole pool is in particular format).
See: https://www.dropboxforum.com/t5/Error-messages/Dropbox-clien...