This is because the sun shines so brightly that we can't see stars. But at night time the moon light is not that bright and hence we do see stars mostly at night.
Because of the brightness of sun we cannot see the stars in day but in night due to darkness star lights reaches to our eyes and we can able to see them.  
stars are present in day too but we cannot see them because of bright light of the sun . in night , the moon light is not too bright because it reflects the light which it gains from the sun . so , we can see stars at night easily .