It's an intentional security thing. If the sftp subsystem command failed OpenSSH could fall back to scp automatically, but there are some security issues with t

It's an intentional security thing. If the sftp subsystem command failed OpenSSH could fall back to scp automatically, but there are some security issues with that. Certain semantics, e.g. file name expansions, differ between scp and sftp on the backend, with the potential for some (theoretical?) exploits. IIRC, in particular, scp is invoked on the remote server through the shell, passing file names and commands as parameters, with shell expansion occurring. Whereas sftp file names and commands are issued in-band to the remote sftp subsystem using an additional protocol layer. (In theory the sftp protocol could be wrapped in TLS; it's independent of the SSH protocol itself.)