It is not because it is a diesel engine, though diesels are more likely to be designed that way.
The engine is so called “flat rated”. That means its power is not limited by simply not being able to burn any more fuel at given piston volume and RPM, but by the peak pressure or temperature.
The controller limits the amount of fuel that can be injected at density altitudes below 9000ft to prevent exceeding those parameters and damaging the engine. Without that the power curve would have the same slope as for the IO360, but the engine would have to be much heavier.
But you don't actually need that power at low altitudes. Drag is the same at the same indicated airspeed, you need the same thrust to maintain your optimal cruise speed at any altitude. But power is thrust times velocity, i.e. true airspeed, so at higher altitude you need more power to maintain that indicated airspeed. So by flat-rating the engine it can allow cruising higher without making it much heavier.
Now diesel engines are more likely to be flat rated.
Only turbo-charged engines tend to be flat rated. Flat rating basically means limiting the manifold pressure. When the limit is higher than normal atmospheric pressure, it can easily be enforced by adding some waste-gates in the turbo that bleed of some air if the pressure was to exceed the limit, protecting the engine from damage. But the atmospheric pressure can always get in the engine, so the engine has to always has to withstand that, so flat-rating normally aspirated engines to below sea level pressure is not done.
Diesel engines are more likely to be turbo-charged. Diesel engines inject fuel only at the point it should start burning, so they can have high compression ratios, and in fact need high compression ratios to reach the auto-ignition temperature of the fuel just by compression. On the other hand spark-ignition engines have the fuel already mixed in the air, so they must not reach that temperature, which limits their compression ratio. And turbo-charging increases the effective compression ratio, so spark-ignition engines can only be turbo-charged so much as the intercooler can prevent the temperature getting too high, while diesels can be turbocharged as much as the cylinders and built to handle.
So because adding turbocharger increases the power with less weight than making the engine bigger, all diesels are turbo-charged these days and have been for quite a while. And adding a bigger turbo to an engine is relatively small change that allows pushing the maximum power to higher density altitude, so once the engine is turbo-charged, it's the logical next step.
I would add that the big aircraft engines of 40s and 50s were all flat-rated, often to over 15,000 ft. I.e. their power curve had similar shape to what they show for the AE300 even though they were spark-ignited engines.
E.g. the huge Wright Cyclones were limited to 54 inHg manifold pressure for take-off and 49 inHg manifold pressure continuous, but they'd go well above that if you firewalled the throttles at sea level. And they had a second stage turbo-charger that the flight engineer only turned on above around 8,500 ft. At the time there were no electronic engine controllers, so the crew had to pay close attention to the manifold pressure gauge to avoid damaging the engines.