There was no Barbarian class in BG1 when Minsc first appeared, so making him a barbarian wasnt an option back then.
AD&D and D&D5 are in many ways extremely different rulesystems, so what Minsc was in BG1 and BG2 could never translate well to BG3 anyway.
I dont really get the fuss about Minsc anyway. You literally only get him towards the end of the game.