Mediation is a problem solving path to resolving conflict. "Conflict" generally means a problem and problems are difficult topics of conversation. Therefore most REALTORS do not proactively discuss mediation with their clients because to do so means broaching the subject of potential problems. No one wants to discuss the fact that something might go wrong. However, the fact that "stuff can happen" is a reality of real estate transactions.
Consider a REALTOR's fiduciary obligation to their client: obedience, loyalty, disclosure, confidentiality, accountability and reasonable care/diligence. Combine that with a REALTOR's ethical obligation (Article 2) to avoid misrepresentation, exaggeration or concealment of pertinent facts and it should be clear that difficult conversations about potential conflict are part of the REALTOR's duty to their client.
A discussion about mediation opens the door to the issue of potential problems while also providing the pathway to a solution. Mediation is a means to an amicable end of a negative situation. Talking about it provides a means to lighten the load of a hard conversation. The client will likely appreciate your honesty and be better prepared to face the problems, should they arise. The knowledge that a problem solving mechanism is in place clears the path to focus on moving forward in the transaction.