I doubt you are going to find many "pro-canonical" Biblical scholars who are going to be interested in participating with a TV special that hypes a work that contradicts the traditional Bible.
You would be able to find plenty.
The reason why they were not included was that they are not invited - because any Christian scholar would be able to blow holes in these heterodox arguments wide enough to drive a Mack truck through.
I'm not a scholar, although I have a far superior theological background to Pagels' and also am more knowledgeable about the source languages than she is. I would have loved to have been able to get a few zingers in, and I doubt that I am alone.