Also because typically with a turbocharger the compressor map is very narrow flow wise at high boost. Too little, or too much flow and the charge temps go way up. At lower boosts it's not as much as issue.
6000 rpm is a bit high though considering it's a 2.3 L, and the max hp isn't that much higher. The Viggen hillclimb car's max torque is at 6500, but then it's max power is at 8000 rpm and it sounds like it revs well past 9000!!