If the orbital plane of the moon and the Earth's orbit coincided, then a lunar Eclipse could be observed every month at the full moon. However, the plane of the orbit of the moon is inclined to the plane of the near-sun orbits the Earth. The angle of this inclination is small - only 5°. But it is enough that in the moon the Moon were slightly above or below the Sun and full moon in earth's shadow falling on the moon disc. Only when at the time of the full moon, all three celestial bodies - the Sun, Earth and Moon line up, eclipses occur.
A lunar Eclipse
Even in ancient times was identified time period through which repeats the sequence of all eclipses. This time interval is called the Saros. The duration of Saros 18 years 11 days and about 8 hours (6585,32 days). During the Saros is 43 Sunny and 28 lunar eclipses, 15 of them are private and 13 full. Thus, each year occur at least 2 lunar eclipses. The greatest number of lunar eclipses in a year is 3. But there have been years when lunar eclipses do not happen at all.
As you can see, lunar eclipses occur less frequently than solar. And yet, lunar eclipses can be observed more sun! This is because the Moon, plunging into the Earth's shadow, is no longer visible throughout the half of the Earth, not the Sun. So every lunar Eclipse visible on a significantly greater area than any sun. And the shadow of the moon on the Earth during a solar Eclipse can be no more than 270 km This spot covers only a negligibly small fraction of the earth's surface.