Yeah, he was down from 245 in the 74 O, and 232 in 75 (Pumping Iron) to only 225 in Total Rebuild. He was still in incredible condition, hard as a rock, and the best guy up there that day (Mentzer had no chest development, I don't know why he bitched so much). I would say the lowered body weight, and the fact that he no longer had the desire for bodybuilding anymore.
Joe Weider, Franco and the dir. of Conan convinced him to do it about 3 months before the show. They all thought it would be good for magazine sales, and publicity for the movie (Conan star fresh off Olympia win).
Either way, himself or not, he won, and deserved to do so. The rest is history....