Here's why:
* Circumpolar constellations are those that never set below the horizon as seen from a particular location. This is because they are close enough to the celestial pole that they appear to rotate around it.
* The celestial poles are directly above Earth's geographic poles.
* At the equator, you are equidistant from both poles. Therefore, no star is close enough to either pole to remain above the horizon for the entire night.
So, while someone standing at the North Pole would see all the constellations of the Northern Hemisphere as circumpolar, someone at the equator would see all constellations rise and set throughout the year.
