It has to do with the angle by which the sunlight is striking the earth. The more you move towards the poles the more obliquely the solar radiation strikes the earth resulting in a decrease of solar radiation per square meter and, consequently, less heating of the surface by the sun.

Dr Randy Boddam thank you for your contribution to the discussion

Sunlight hits a smaller surface area at the Equator so heats up quickly compared to the poles. There are fewer atmospheres to pass through at the Equator compared to the poles. This means more heat from the sun makes it to the surface of the Earth. The Sun's rays strike Earth's surface most directly at the Equator. Near the poles, the Sun's rays strike the surface at a slant. This spreads the rays over a wide area. The more focused the rays are, the more energy an area receives. Due to the spherical shape of the Earth, sunlight falls on different parts at different angles. Direct and focused sun rays falls on the equator and hence, the regions here are hotter and warmer. When the sun's rays strike Earth's surface near the equator, the incoming solar radiation is more direct. Because the Earth is a sphere, the surface gets much more intense sunlight (heat) at the equator than at the poles. During the equinox (the time of year when the amount of daylight and nighttime are approximately equal), the Sun passes directly overhead at noon on the equator. The equator receives the most direct and concentrated amount of sunlight. So the amount of direct sunlight decreases as you travel north or south from the equator. Because Earth is a sphere, not all part of the Earth receives the same amount of solar radiation. More solar radiation is received and absorbed near the equator than at the poles. Near the equator, the Sun's rays strike the Earth most directly, while at the poles the rays strike at a steep angle. The tropics receive a great amount of direct solar energy, which produces more evaporation than higher latitudes. The warm, moist air rises, condenses into clouds and thunderstorms, and falls back to earth as precipitation. The Sun's energy is generated by nuclear fusion occurring in its core. Here, protons of hydrogen atoms violently collide and fuse to create a helium atom. Countries, towns and cities located around the equator experience hot weather throughout the year. It is because the sun remains almost directly overhead every day. Countries that are further north or south of the equator experience a change in seasons, when hot weather follows cold weather. The climate near the equator is usually hot and humid because the sun's rays hit the earth directly at this latitude. The seasons are also less pronounced near the equator because the earth's tilt is not as pronounced. Sunlight hits the Earth most directly at the Equator. The curve of the Earth means that sunlight is spread over a wider area the further you move from the Equator. Sunlight hits a smaller surface area at the Equator so heats up quickly compared to the poles.Due to the Earth's natural bulge where the equator is located, the equator is closer to the Sun than any other place, this also results in higher temperatures all year round.