When removing the case cover, you will see two sets of wires: a single cable that's connecting the metal case and the PCB and also a bundle of six thin cables which are coming from the cartridge and are carrying its signals to the preamplifier.

Notice that this bundle of wires is quite long and runs unshielded from the tone arm to the circuit board.The signals that are carried by these cables are naturally very weak, only a few milivolts (believe me, this is a very low level signal). Naturally, this audio signal is very sensitive to all kinds of interference.

The metal case shields the upper side of the board from electromagnetic interference, but the bottom is directly attached to the wooden plate, and wood is not a very good conductor ;)