Trismegistus

I think it's because the card actually was a juggler in the old Marseilles Tarot decks, and the unnumbered card was the Fool ("Le Fol" or "Le Mat"). Then, European occultists "corrected" this to the Magician when they "rediscovered" Tarot's secret origins and meanings in the 19th century. Stuff like this is one of the things I find really fascinating about the cards.