Because its orbital velocity at different times of the year is not constant. Besides, elliptical orbits are what come out of planets moving in 'inverse- square' force fields such as the gravitational field. Circles are a very special case, and in fact, the orbital eccentricity is so close to 1.000 that the Earth's orbit is practically circular. But, of course, 'practically' is not good enough at the scales we are talking about!