Forums are not run out of the kindness of people's hearts. They are an advertising platform - people make money from them. OR they have an agenda. (Sort of like Facebook - you are not the customer, you are the product.) As such it would probably not be in either forums interest to merge.
The other forum in question must have an agenda then. Care to speculate?(no ads and an uncontactable owner- over a period of years. No real amount of traffic either at this stage)