Because there's no support on this roster. There is no one else to pick up slack. You have Kaprizov, who is a special, special player, among the best in the world.
After that, there's Boldy and Rossi who can really impact play in a positive manner, but can't do it by themselves. There are very few that can.
For the record, all these "difference makers" that keep getting brought up, they all have tremendous supporting casts with them, and most of them are not the primary guy. Both Tkachuk's are being supported by other players who are better than they are, and they have deeper secondary scoring to pick up slack when they go cold. Mikko Rantanen is closer to a Kaprizov level player, but even he is the third best player on his team.
Most players in this league need help to produce offensively. You can't just put them on a line with Marcus Johansson, Frederick Gaudreau, and Ryan Hartman and expect them to produce night in and night out.