There are a few problems with only having two pitches. One is that hitters have an easier time adjusting to your pitches if they’ve already seen them that day. That’s a big part of what the times-through-the-order penalty is about, and relatedly, it’s why guys who can’t develop a third pitch end up tracked to the bullpen.
Having only two pitches leaves you a thinner margin for error. Take a pitcher like Yu Darvish, for example, who throws seven pitches. He and his catchers and coaches come up with a game plan for how to attack opposing hitters according to the hitters’ weaknesses.