According to wikipedia "When electricity was first introduced into the household, it was primarily used for lighting. At that time, many electricity companies operated a split-tariff system where the cost of electricity for lighting was lower than that for other purposes. This led to portable appliances (such as vacuum cleaners, electric fans, and hair driers) being connected to light bulb sockets. However, as electricity became a common method of lighting houses and operating labour-saving appliances, a safe means of connection to the electric system other than using a light socket was needed. The original two pin electrical plug and socket were invented by Harvey Hubbell and patented in 1904."
quoted from web page http://en.wikipedia.org/wiki/Power_plug
The Edison light bulb socket was standardized before the electrical outlet was, and the house builder probably didn''t know which power plug configuration would win, as the NEMA 1-15 did. Today, no safety committee would accept the Edison light bulb socket if it were a new proposal (you can get your finger inside it and touch both sides of the line).