Slowbeef was the straight man. Diabeetus was the funny man. Seldom was there a video where Slowbeef wasn't opposite to a funny man like Proteus versus Diabeetus playing opposite to another funny man. Maybe it was intentional, maybe it was accidental, but you can't have two funny men because comedy can't thrive without juxtaposition, and they avoided this. A funny man without a straight man is just a clown, and a straight man without a funny man is just an average, unassuming person.
So when Slowbeef streams, he's just an average, unassuming person. Nobody's going to tune in for that. The droll commentary and underperforming gameplay are just accents to it.