The solar eclipse achieves totality only in limited geographical regions because
1
the earth is not a smooth flat surface, but has elevations and depressions.
2
the size of the shadow of the moon on the earth is small compared to the cross section of the earth.
3
the trajectories of the earth around the sun and the moon around the earth are not perfect circles.
4
sun rays can reach most of the peripheral regions of the shadow of the moon due to a atmospheric refraction.