Many non-profit organizations have a hard time getting enough good board members. So of course they would prefer if their board members stayed for as long as the term limit policy allows. My opinion is that a board member will stay on a board if they feel like they are accomplishing something worthwhile.
Everyone has things to do, the stuff we choose is up to us. Most people don’t go looking for unpleasant situations, so if your board meetings are nasty, you will lose board members. If your organization cannot figure out what it is doing or actually accomplish anything, you will lose board members.
Do you keep track of how long board members stay on your board? Do you have these metrics? It might be a good idea to talk to board members who have left and find out why they made that decision.