There was a sense of "wasting a port". A modern Linux /etc/services has only 200 or so reserved TCP ports (out of a possible ~50k) so that fear might have been

There was a sense of "wasting a port". A modern Linux /etc/services has only 200 or so reserved TCP ports (out of a possible ~50k) so that fear might have been overblown. I suspect the bureaucratic overhead of needing to go to IANA to reserve a new port might have had a chilling effect. See: https://www.iana.org/protocols/apply https://www.iana.org/form/ports-services