Why does the earth rotate

Well, the earth rotates because it has an axis that it spins on. If that's not the answer your looking for, I got you. The reason why the earth rotates is because it's angular momentum

Angular Momentum. The earth formed out of a nebula that collapsed. As the nebula collapsed it began rotating. The Earth's rotation comes from the initial tendency to rotate that was imparted on it when it formed, only the relatively weak tidal forces from the moon act to slow it down