Because that would be closer to what the developers originally planned and similiar to what other developers have done. The game used to have 3 skills for intimidation, charm and reasoning BTW before it was ripped out.
But charisma only does that while strength and intelligence have other purposes. From a maximization standpoint you would never ever get charisma if you could use strength or intelligence to win arguments.
I can plainly see why they condensed it all into one skill. Sometimes you want to use a different line of dialogue. Why force you to
always be a kissass because you picked x instead of z or y?
