Quoted from Rarehero:incredible reach.
I agree. While the Terminator franchise is huge (and they have been aggressively licensing 'Genisys'), the new film is totally unproven and as such is not the sort of risk a company with two pins a year gambles on - especially seeing as the last one was a critical disaster and not a box office home run either. *cough* *cough* episode 1 *cough*.