Because the Moon's orbit around Earth is tilted.
This video explains how our moon creates a solar eclipse, why it's such a rare event to see, and how data from NASA's Lunar Reconnaissance Orbiter has enhanced our ability to map an eclipse's path of totality.
On Monday, August 21, 2017, all of North America will be treated to an eclipse of the sun. Anyone within the path of totality can see one of nature’s most awe inspiring sights - a total solar eclipse. This path, where the moon will completely cover the sun and the sun's tenuous atmosphere - the corona - can be seen, will stretch from Salem, Oregon to Charleston, South Carolina. Observers outside this path will still see a partial solar eclipse where the moon covers part of the sun's disk.