TOS doesn't tell us much interesting, in dramatic terms, about Kirk as a young person. There's a plethora of throw-away detail about events that happened to him - like the Kodos business - but what's said about him is stuff like "a stack of books with legs." Oh, and he blamed himself for what happened to Garrovick. While those two or three events might function as parts of the armature of a story it's not one that anyone would try to build a big-budget tentpole feature around, and those events give us nothing in terms of a dramatic arc and in no way illuminate his conflicts and idiosyncracies. Such things had to be made up entirely for this movie.
The exception there, of course, is the Kobyashi Maru cheating incident - and the movie used it in a far more effective way than any of the fannish speculation or previous attempts to dramatize it in narrative have.
