There are two factors. First, the rotation axis of the Moon is inclined by a few degrees to its orbit, and this orbit is itself inclined by a few degrees from the plane of the solar system. Second, the Moon is so close to the Earth that its parallactic shift is quite significant as seen from the surface of the Earth. This allows both a shift in latitude and a shift in longitude to occur so that we actually get to see more than 1/2 of the surface of the Moon from the Earth. In fact, we see about 59 percent of the lunar hemisphere during the course of a lunar month.