Right - there are different forms of applause used in other cultures or in previous time periods. Things like snapping or stomping might also be used to show appreciation. Hand clapping is just kind of what has become most common.
Maybe because one of the first things we do with little babies, in the learning process, is hand clapping and smiling when something good/fun happens. It's easy to learn. Whistling and slapping one's thighs are more difficult to learn.