Presumably for roughly the same reason in RE. It has a wider orbit than Venus and Mercury. Also known as, it doesn't come below the sun. Why? Hell if I know, the FE cosmology is already pretty screwed up to be honest. But remember the moon phases come from a shift of only a few hundred miles up and down, if that.
The only thing that does not make sense with that answer is the idea that some planets could have wider orbits than others. As alluded to in my thread about An Experiment Everyone can Try, all the planets, the moon and the sun (and the zodiac constellations) have the same width of orbit. They can all only be seen directly overhead for people between the Tropic of Cancer and the Tropic of Capricorn and every one can be seen directly overhead at one time or another anywhere between these extremes. People on the equator are able to see one of the zodiac constellations directly overhead at least once a night and during the two equinoxes, they get to see all of them over the two nights at some time directly overhead. Since all the zodiac constellations, the moon, the sun and all the planets occupy the same overhead space, and all their orbits follow the equator during the equinox, it really makes explanation for phases hard to come up with.
If you are looking from an RE point of view, the planets are basically on the same plane as the earth, but a wider orbit means that they are further out in space than we are in their orbit round the sun. Imagine they were all balls on a piece of sting spaced at different distances along the string. If you then whirled the string around your head, that would be similar to the planets going around the sun with the ones nearest the end of the string being the further out or on a wider orbit. The difference with the planets is that unlike the balls on the string, they are orbiting at different speeds, so the outer ones take much longer to complete an orbit.
So to get back to the phases, planets that are further out from the sun than us, will always appear to us to be lit fully by the sun, but planets nearer the sun than us will sometimes have the side that faces us in darkness or partial darkness because the area facing the sun will be lit.
If you want the answers from an FE point of view, you will need to ask an FE believer because I can't see how it would work if the sun is going round a flat disk earth.
Roger