If the Moon's orbit around the Earth were in exactly the same plane as the Earth's orbit around the Sun, we'd have a total solar eclipse every month (but 100% totality would be seen only from the tropics).
But in fact they're not in the same plane. The Earth's spin axis is tilted by about 23 degrees relative to the Earth's orbit around the Sun, and the Moon's orbit is closely aligned with the Earth's spin. As a result, the Sun and the Moon do not follow the same path in the sky.
We get a solar eclipse only when (a) there's a new Moon, so the Sun and Moon are in the same position east-to-west, and (b) the new Moon happens when the Sun and Moon happen to be closely aligned north-to-south.
Since the Sun and Moon are both about half a degree wide (as we see them in the sky), the 23-degree offset of their paths makes solar eclipses relatively rare events.
Lunar eclipses, which occur during the full Moon when Moon passes into the Earth's shadow, are more common because the Earth is bigger than the Moon, and so has a much wider shadow.