While I'll agree Stevenson had a great career, he never faced anyone his size who could dish out punches as well as take them.
The style of wading straight in, looking for a straight right knockout punch is limited, at best, at the higher levels.
Look at Frazier. He had that style, had to adopt a bob and weave to avoid getting hit. Stevenson was all offense, little defense. Most heavyweights had great offenses. Once the heavyweights see Stevenson's style, they can counter a steamroller heading straight in.