According to Peter H. Brothers in Mushroom Clouds and Mushroom Men, "Nick Adams was unfortunately unavailable to reprise his role." I think that's probably the case but Brothers doesn't cite his source.
Now why was Adams unavailable? Beats me. A quick glance at his IMDb page shows he made only one film released in 1966, Don't Worry, We'll Think of a Title, and he is uncredited in that. (Probably a small part.) He also guest starred in several TV shows during the '66-'67 season. All of those were first broadcast from October 1966 to February 1967 so he would have spent the tail end of 1966 working on those. WOTG was released in Japan on July 31, 1966, so it seems doubtful that those productions kept him from doing a movie in Japan in early 1966.
And for what it's worth, the IMDb trivia page for Gargantuas suggests that Tab Hunter was the first choice for Dr. Stewart.
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