Because at the coarser pitch you will not be developing full power....for one thing the engine driven fuel pump will be turning slower pumping less fuel. Additionally, because these engines are fixed timing the slower RPM will cause the spark to happen closer to TDC and the peak power pulse in the combustion chamber will not have the leverage to apply a maximum push against the piston. This will cause horrendous heat and pressures in the, effectively smaller, combustion chamber and you risk severe detonation which can destroy your engine within seconds.