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Because the torque is dropping faster past the peak. Try looking at some hp and torque charts. Hp=torqueX rpm/constant(5250). Hp almost always peaks above the peak torque rpm and the lines cross at 5250. The torque curve is a proxy for VE, it starts out low and climbs to the peak (say 90%), then drops off, sometime dramatically, after the peak torque rpm. If you can keep the slope of the torque drop low, you will make more power. Engines with fixed valve timing must balance low speed performance with high rpm breathing. Engines with ideal valve timing for high rpm breathing are lousy performers at low speed. Variable valve timing was invented to improve the compromise. You can pretty much tell an engine's power band by focusing on when the intake valve closes.
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Paul
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