Aside from those who thought it was an awful story idea, I wonder if another reason the idea was passed on was because the assassination was still too fresh to use as a plot point in a film like this? Especially if it was first pitched in the '70s/early '80s?
Partly. Shatner mentioned people also feeling like the story didn't leave much room for imagination because everyone knew what the ending had to be: Kennedy must die in order to save the world. There was also the matter of reconciling the story itself with the events of previous films. How could Spock be the shooter in Star Trek III
if he died in TWOK, for example?
I think the JFK storyline had potential, but I think it would have been disappointing for fans wanting an original idea rather than something already used in "The City on the Edge of Forever." Plus, Nimoy was determined to make a lighter film for TVH after the drama of TWOK and TSFS. JFK dying on the big screen wouldn't have been that. Unless Jack Ruby shouted "double dumbass on you" instead of "you killed the President, you rat" before shooting Oswald.