why are iron rails of railway track fixed over wide wooden or concrete sleepers

Because the area of contact will be more and the pressure exerted by it will be lesser in the ground and don't sink into it.

Because the area will be more and the pressure will be less and don't go inside or we can say they do not sink

Iron rails are fixed over wide wooden or concrete sleepers to reduce pressure received from the train  by increasing the area of contact, because if surface area increases pressure decreases and vice-versa, they also transfer the loads to the ground underneath. They have one more role which is to hold the rails with correct width apart.