The related contacts on a listing record are the contacts that are involved in the Sale - so the Owner and Buyer. This allows them to be quickly accessed for merges.
As the tenant is not part of the sale, they're a related contact of the Property Record and not the Listing.
If you wish to have a reference to them on the listing record, you can add them to the Key Details tab under Office Use Only > Notes. This will also include them on the Internal Stocklist.