Because SFTP is a better fit for simple (but secure) file transfer, WebDAV handles a lot more, but it's overkill for most of the things FTP was used for.
OTOH, AFAICT, SFTP has largely replaced FTP for the system-to-system role (though ad hoc HTTP-based protocols are also common.)
OTOH, AFAICT, SFTP has largely replaced FTP for the system-to-system role (though ad hoc HTTP-based protocols are also common.)