Why do planets follow an elliptical orbit? Why not a circular orbit?

Elliptical orbits are caused by aggregate gravitational effects of the Solar System itself. Each body in the system will exert gravitational forces on all the other bodies, including the Sun. This causes the Sun to actually "wobble", it does not sit still in the heart of the system. Therefore, the center of mass of the Solar System does not sit with the Sun, it sits a slight distance away. This center of mass is called the barycenter, and this is one of the foci associated with the elliptical planetary orbits.