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