And the answer is DRAG also known as useful load
If the indicated parameters are equal, i.e. airframe and rpm, then for any given airspeed the power required for flight is equal. However, as drag in the form of extra pax, extra fuel, extra baggage, are added to the equation then additional POWER is required to maintain rpm and overcome the drag.
Don't forget, the engine requires power to turn the prop at rated rpm, 2550 is given, and also provide thrust to move the inherent drag to a speed sufficient for flight.
In the example given, 30hp is available to bring along additional drag.
Mr. Piper worked this out well with the Cherokee 140, 180 and 235. The 140 can just about lift 4 people and not much fuel, the 235 can lift its own weight. The airframes are similar.
Horses for courses.
Simple.
Sultan Ismail