Because sunlight has maximum power when it strikes a surface at a 90° angle. The intensity of sunlight hitting Earth surface can vary widely because of clouds, sand, dirt, ash and pollutants in the air. Its is the POWER that is important.
Sunlight can strike the Earth's surface at a 90° angle only between the latitudes of +23.5 ° N, the Tropic of Cancer, and -23.5° S, the Tropic of Capricorn. Any latitude north or south of that, sunlight strikes the surface at an angle , so sunlight has less POWER. This is WHY satellite dishes and some telescopes are tilted at an angle.