Well I think I can speak for most people when I say I think of assassinations as a slight pause in my gameplay that constrains me from 'whoopin' some tail' faster. Whenever I assassinate someone I'm sitting there DIEING to move on with the game. The thing about assassinations is is that while being cool looking, #1 they don't stylistically fit in with any other Halo game. #2 Because there can't be an unlimited amount of different assassinations, they get old. Fast. #3 If a player gets put in a bad or vulnerable situation because of choosing to assassinate someone, there's no option to cancel it, leading to guaranteed death.
Also, with Master Chief being the only spartan in Halo 4, never before needing or having a knife, AND he is supposedly fighting forerunners, it would make 0% sense for there to be a knife anywhere near Chief.