If I recall correctly, there is something called roughly "ignition delay", meaning you need some time for the fine spray of fuel to start vaporizing AND physically "cracking" the long hydrocarbon chains by heat and pressure in the cylinder, before it can burn properly/ideally/equally everywhere.

This is possible to achieve under 4-4.5k rpm, but further increasing revs means there would not be enough time for this process during each combustion cycle.

I am sure my technical terms are wrong, I only read about it in my native language, so maybe someone will correct me, but a very smart guy was explaining it roughly like this.