I think it was the latter. I was amazed at how many households had a picture of JFK on the wall. So Teddy gets many votes out of respect for his dead brother. Then there is the seniority thing and his ability to do things for the state, whether or not he actually does anything. People have a hard time letting that go.
Ultimately though the same people in MA would would criticize SC for keeping Strom Thurmond past time or those in WV who demand Robert Carlyle Byrd see nothing out of the ordinary in retaining EMK forever.