Why wasn't Dave Prowse interviewed ?

Dave Prowse and George Lucas had a pretty legendary bitterness towards each other that seems to have really peaked during production of Return Of The Jedi where Lucas believed that Prowse leaked stuff to the press, despite Prowse always having denied doing that. And Prowse believes that Lucas unjustly cast Sebastain Shaw for the big Vader unmasking at the end of Jedi behind his back, and as a way of getting back at him, and has stated that he was originally promised by Lucas that he would get to do that part of the role. This wasn't the first time the two had issues, but it seemed to be the final straw that broke the camels back between the two men once and for all.

On the original Star Wars Prowse was never told that his voice was to be replaced by James Earl Jones, only finding that out when he saw the film for the first time, and he held a grudge over this, particularly over the fact that he felt that Lucas had hidden his intent to replace his voice from him deliberately, and he has said that he found that whole experience humiliating. Likewise, during Empire Strikes Back Lucas gave Prowse a script with false dialogue during the scene at the end between Vader and Luke where Vader reveals that he's Luke's father, so that Prowse didn't know the father/son twist. Prowse, again, resented Lucas for not trusting him with the real script, and instead having him deliver false lines (that would later be dubbed over once again by James Earl Jones) only for Prowse to once again feel humiliated when he saw the final film and learnt that he hadn't been trusted with the film's true ending. And the thing that did him no favours at all in the eyes of Lucas is that he was always very vocal about these feelings, which pissed Lucas off, and according to Prowse, Lucas saw him as being nothing but yet another ungrateful actor that wasn't worth the effort to treat with any level of respect.

Prowse also had a habit back in the day of signing autographs with the term "the real Darth Vader", which allegedly also irritated Lucas, although these days usually he just signs them with his name and "Darth Vader".

So, yeah, neither one likes the other very much, and both have their share of issues as to exactly why that is, but Lucas has pretty much frozen Prowse out of everything official to do with Star Wars for the past twenty odd years, even refusing to have Prowse come back for the final scenes of Sith, despite Prowse petitioning for the chance to do so. Lucas apparently wouldn't even meet with or talk to him about it.

So, yeah, given all that it's not really all that surprising that he wasn't on the doco, and unless a miracle happens and the two mend fences, I highly doubt that you'll see Prowse involved in any capacity on anything official (i.e. Lucasfilm approved) to do with Star Wars, at least while both men are still alive anyway. Pretty sad, but there you go.