Therin of Andor wrote:
Don't forget that, as with Decker, Ilia and Xon being the new crewmembers who would carry "Star Trek: Phase II" if/when Shatner dropped out after 13 episodes, Saavik and David were thought of as characters who could carry a series of Spockless telemovies if/when Shatner's regular movie career took off, as he'd hoped.
Just a speculation, but could it be that Alley'[s contract included an option for more telemovies (and even telemovies that get promoted to the big screen, as ST2 was), but not big screen sequels?
I'd think it unlikely, but it's the sort of legal technicality that agents use as bargaining power. Something similar allowed Jeffrey Hunter to leave after The Cage (he was optioned for a series, but not a second pilot).