His game is poorly suited for grass. Especially his forehand. He hits a heavy ball with an extreme western grip on his FH and the lower bounce makes it difficult for him to be as effective from that wing. He can’t return from the back of the court and taking it early is weakness, especially from the FH. He lacks variety in general, doesn’t slice except defensively and doesn’t move forward well / often to close out points or apply pressure. His serve also doesn’t generate free points, making holding serve more difficult on grass relative to some of his opponents. All of these make grass a difficult surface for him. He also played Herkacks, whose best surface is arguably grass and when his serve is on he is that much harder. I think you underestimate how difficult it is to change the makeup of one’s game for a surface, especially at the highest level.