What is the primary reason that solar and lunar eclipses do not occur every month?

Solar and lunar eclipses only happen when the Moon, Earth, and Sun line up at the same time, but the Moon’s orbit is tilted about 5 degrees to Earth’s orbit around the Sun. Because of this tilt, the Moon usually passes above or below the Sun’s or Earth’s shadow, missing the eclipse path. Eclipses can only occur when the Moon is near one of the two points where its orbit crosses Earth’s orbital plane—called the nodes—so the alignment is perfect. For example, when the Moon is at a node and also on the far side of Earth, a lunar eclipse can happen, but this alignment happens only a few times a year, not every month. Thus, the orbital tilt and the need for precise alignment prevent monthly eclipses.