Because without a nitrous controller when you press the button you are spraying a set amount of nitrous regardless or RPM. For arguement say you are spraying 1# per minute. ( A 100 shot on a V-8 uses 1# per minute, a 25 shot on a 4-cyl. bike will use a lot less) Now at 4000 RPMs each cylinder is consuming 1/1000th of a pound per revolution, at 8000RPMs each cylinder only has to consume 1/2000th of a pound per revolution. If you spray at 8000 vs 4000 you only have half as much nitrous in the cylinder, thereby giving you lower cylinder temps.

The less nitrous in a cylinder the less power but the cylinder does not run as hot and you reduce your risk of explosion.