Imagine you're rotating on a merry-go-round. When do you feel that you are moving at a higher speed, on the edges or closer to the centre? "Of course, near the axis!" - you would say. The same about Earth: the equator is far away from the axis, so it has greater speed. The poles are close to the axis, so the speed is smaller there. It comes from a simple physical principle: the further you are from the point of rotation, the faster you move.