Because they make these movies for a wide audience and the characters are the best known to most people. Also for the most part it worked rather well. I don't think they needed to fully redo the ending to TWOK. But watching it with people who aren't really fans or even aware of most of Trek, it works. Especially with Kirk's "death scene". It showed that Kirk has finally grown up, he's willing to die to save his crew without hesitation. Also Spock's reaction was important character development, he's over his obsession with death completely and has figured out that Kirk really is his friend. It could have been handled differently, but it works especially with people who haven't seen TWOK a million times. Their reaction to Spock screaming Khan isn't laughter, but saying that Spock's pissed and about to kick some genetically engineered ass.