Our friends at "Words to the Wise" say... " it arose from the game of bridge, in which the suit of spades is the highest ranking suit in the game. In spades dates in the written record from 1929 in the U.S.
Spade used to refer to the card suit dates in writing from 1598. It derives from Italian spade "sword". It is simply a coincidence that the spade symbol in British and U.S. playing cards (which came ultimately from France) resembles a spade or shovel."