ANSWER:
We don't get an eclipse all period because the Moon's revolution is turned at approximately 5∘ to the ecliptic.
EXPLANATION:
If the Moon's revolution was in the equal level as the Earth's we would encounter two eclipses all period. A solar eclipse would appear every new moon also a lunar eclipse would appear every full moon. As the Moon's orbit is turned at 5∘ to the ecliptic, an eclipse can simply happen when the Moon is near one of its links which are the points where the Earth and Moon circles intersect. The line joining the links also has to be facing towards the Sun.
