why is it impossible for someone on earth, even with a powerful telescope, to see the surface features that cover the whole surface of the moon?

Only one side of the moon faces the earth at all times as it revolves around the earth. This is because the Moon rotates on its axis at the same rate that the Moon orbits the Earth - This is referred to as synchronous rotation. To be able to view the other side it is only possible by use of probes or space satellites.