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 a Sale Listing record, you can add a note and pin it to the top of the Stream.
Take a look at the following help article for instructions on pinning notes in the Stream: The Stream Tab.