Why is it called missionary position?

The missionary position is often referred to as the lovemaking position, and is performed with the man on top of the woman, and the woman most often wrapping her legs around the man, or propping him up or doing something else.

When you think about sex positions, you'll find that the traditional missionary position is the one that's used most often, and you're probably one of the people who use it. For some, this position is not as interesting as others such as rear entry and butterfly, but it does have its own advantages. The biggest advantage is the positioning and orientation of the body, which makes for a more intimate sexual experience.

Expanded:

In this position, there isn't a whole lot for the woman to do, but that doesn't mean that the woman is just laying there and not doing anything. Anatomically speaking, women don't have to worry about maintaining a proper angle in this male-on-female sex position.

The way to do this is for the man to lie on top of the woman, and the woman most often does nothing more than wrap her legs around the man or prop him up or something else.

Baidu Encyclopedia - Missionary Pose