As to why Bill Maher didn't interview some of the "big guns of Christian thought", I'm guessing that he wasn't allowed the opportunity, just as he wasn't able to interview the Pope or the head of the LDS church. (I won't spoil things by saying how that was disclosed in the movie.)
This is interesting. I know a couple of the people that I mentioned, and all of them have asked for time on Maher's show, and been denied...so again, what's he afraid of? My guess is that since their are intelligent Christians that can not only refute, but embarass Maher he does not want them on his show.