There's solid SFTP clients for every OS (even Android!) and all it requires on the server is the already installed OpenSSH, so I never really saw the need to look into webDAV.
For user-facing oses yes, but webdav is available on lot "embedded" devices like Smart Whiteboard, and for many oses is built in (even Windows 98), so it's easier to deploy
all sorts of network security appliances in foreign networks make reliance on ssh a futile endeavor. webdav works over standard ports with standard tooling (web browser)
