If the Earth were a disk with its surface perpendicular to the rays of sunlight, each point on it would receive the same amount of radiation, an energy flux equal to the solar constant. However, the Earth is a sphere and aside from the part closest to the sun, where the rays of sunlight are perpendicular to the ground, its surface tilts with respect to the incoming rays of energy with the regions furthest away aligned in parallel to the radiation and thus receiving no energy at all.
The amount of heat receive by places far from the equator becomes less because of the tilting of the Earth's axis and the shape of the Earth...
