Roasting, boiling and frying are pretty obvious--you stick your food somewhere violently hot and it's bound to cook. Steaming isn't so obvious at all. It seems, well, wispy. Though steam gets as hot as boiling water, it doesn't pass its heat on as quickly. (On the other hand, scald yourself in steam just once and you'll know it can cook every bit as well as hot water.)
So steaming shows up fairly late in the history of food preparation. Basically, people started steaming food for economic reasons. If you're already boiling something, you can cook something else in the steam at the same time and save on fuel.
The roots of American cookery go back to heavily forested northern Europe, where fuel wasn't a problem. As a result, steaming plays little part in American cookery. (Sure, we steam clams, but that's an adaptation of Native American pit baking.) By contrast with Europe, North Africa has a scanty supply of wood, so it developed couscous, a grain product traditionally steamed over the stew it will be served with. The nomads of Central Asia, who wander the vast, treeless steppe, have always made steamed dumplings called mantu.
In many parts of East Asia, fuel is a very serious problem because rice makes heavy demands on land. China cut down most of its forests centuries ago to make room for rice paddies. In Confucius' time, when wood was still plentiful, long stewing was China's preferred method of cooking, and the symbol of cookery was a big caldron called a ding. Modern Chinese cookery would be symbolized by the fuel-efficient wok for stir-frying and the equally fuel-saving bamboo steaming baskets that fit over it.