What causes the earth's rotation?
The Earth formed at the same time the sun and solar system formed, some 4.5 billion years ago. A nebulae of diffuse matter started to collapse and as it did so this disk retained a preferred spin. As the collapse continued, the spin direction accelerated and initial angular momentum has to be conserved in a closed system. So, most of the planets in the solar system took on the direction of this initial planetary spin, except for Venus and Uranus.
Also, there is very little resistance in space so once an object develops a rotational spin to it, it does not slow down very fast.