I don't blame Capra for quitting. Any film that paired a screenwriter of dubious talent like James Edward Grant with his longtime buddy John Wayne was bound to be bad.
Wayne, Grant and director Henry Hathaway were all men of great talent, but by 1964 they were getting lazy in much of their work, especially when they worked together. Hathaway reportedly told Sam Bronston that when people went to a John Wayne film they expected action and "tits" and that Grant knew exactly what to write for such a movie. Basically they went through the motions on a film that wasn't anything special to begin with, one reason it was such a flop.
Yes, that's right; my mistake. I also read that in a bio of Samuel Bronston. Same difference, though. They just ground out a movie suited to what they deemed Wayne's fan base. A different writer and perhaps director would have turned out a different movie, and perhaps a better one.