Copper is; unlike iron and steel; not ferromagnetic, but diamagnetic. This means that induced magnetic fields in copper will counter the applied force.
When you drop a strong magnet through a copper pipe, the moving magnetic field will induce currents (Lenz’ Law). These currents will now induce their own magnetic field. This magnetic field counters the falling magnetic.
Result: the magnet will fall way slower than if it was falling through a plastic pipe.
