Everything attracted towards the center of earth but gravity is zero at the center of earth.why?

There is nothing special about the center of the Earth itself; the attraction is in that direction, due to the accumulated effect of the attraction from different parts of the Earth.

At the center of the Earth itself, the gravitational attraction towards the left, for example, by some pieces of planet Earth, would be exactly compensated by gravitational attraction towards the right, by other pieces of the planet.