Hi Ali,
The reason is that in a system which is functioning normally the neutral has no, or very little potential with respect to earth.
On a normal supply the neutral side of the transformer is grounded. So the "hot" side of the line is at 120, 240, or some other voltage with respect to the earth. If you touch it while simultaneously in contact with the earth, you therefore have that voltage applied across your body and you feel a shock.
But even though current is flowing through the neutral, the only voltage present on it is a very small amount due to voltage drop along the cable. So you may have just 1 or 2 volts on it with respect to earth -- Not enough to shock you.
