Why is scp impossible and rcp possible?

scp uses 22/tcp as part of ssh (secure shell).
rcp uses 514/tcp as part of rsh (remote shell).

Of course, neither can be used unless it is properly configured on both sides of the client server. ssh is not configured and the remote shell is only configured.

(1) sshd is not installed or sshd is not started even though it is installed.
(2)The well-known port (port 22) used for ssh communication is blocked
If that's the case, I don't think it's possible to communicate with the ssh system including scp.

I think that ssh started to spread around 2000 and there are not many Linux/UNIX environments today, but
There may be environments that were built before that and are left behind.

(I don't know if it's good or bad, so I won't tell you...)