Because there are two types of metals when it comes to megnatism. FERRO-MAGNETIC and NON-FERRO-MAGNETIC. Unfortunately copper is a non ferro-magnetic material.
Lets me explain it in better details, ferro-magnetic materials (Iron, steel, negel, cobal) have these so-called dipoles inside them. when a magnet is put near them these dipoles are forced to look away from the N-pole of the magnet or towards the S-pole. Copper doesnt have dipoles.
DIPOLES: ( http://en.wikipedia.org/wiki/Dipoles )
A dipole (Greek: di(s) = double and polos = pivot) is a pair of electric charges or magnetic poles of equal magnitude but opposite polarity (opposite electronic charges), separated by some, usually small, distance . Dipoles can be characterized by their dipole moment, a vector quantity with a magnitude equal to the product of the charge or magnetic strength of one of the poles and the distance separating the two poles. The direction of the dipole moment corresponds, for electric dipoles, to the direction from the negative to the positive charge. For magnetic dipoles, the dipole moment points from the magnetic south to the magnetic north pole — confusingly, the "north" and "south" convention for magnetic dipoles is the opposite of that used to describe the Earth's geographic and magnetic poles, so that the Earth's geomagnetic north pole is the south pole of its dipole moment. The only known mechanisms for the creation of magnetic dipoles are by current loops or quantum-mechanical spin since the existence of magnetic monopoles has never been experimentally demonstrated.