I’m pretty sure them using Shaw when for unmasked Vader is because he was a seasoned actor who turned in an amazing performance. Prowse was a body actor. Look at his filmography on Wikipedia. I’m not really sure why he was so upset or what he was expecting. Playing tall imposing monsters was basically his thing.
Not saying that in a derogatory way at all. The body acting for Vader by Prowse is masterful. But that’s what Prowse was.