Here's why:

* Cairns' Latitude: Cairns is located at approximately 16.92° south latitude.

* The Tropic of Capricorn: The Tropic of Capricorn, at 23.44° south latitude, marks the southernmost point where the sun can be directly overhead.

* Solar Declination: The sun's position in the sky changes throughout the year due to the Earth's tilt. The sun's declination (its angular distance north or south of the equator) reaches its southernmost point at the December solstice, when it's directly overhead at the Tropic of Capricorn.

Therefore, Cairns is located too far north of the Tropic of Capricorn for the sun to ever be directly overhead.