I'm sure Dave was picked because since this was the first special episode of BITBBH, they would want a veteran performer from the Muppet Show days to make it even more special, for the puppeteers and the puppet geeks like us. Jim and Richard were obviously dead, Jerry was probably busy upstairs doing The Count, Frank was probably off directing... leaving Dave. And I'm sure he was enthusiastic to take part in the show. I've met several of the puppeteers who worked on Bear and they are a really great happy group of people. As is Dave!