Here's why:

* Solar cookers rely on direct sunlight: They use mirrors or reflectors to concentrate sunlight onto a focal point, generating heat to cook food.

* Clouds block sunlight: Cloud cover significantly reduces the amount of sunlight reaching the earth's surface.

* Insufficient heat: Without enough sunlight, the solar cooker won't reach the temperatures needed for cooking.

However, there are some workarounds:

* Solar ovens: Some solar ovens are designed with a glass panel that can trap some heat even on partially cloudy days. However, they won't be as efficient as with direct sunlight.

* Hybrid systems: Some systems combine solar energy with other energy sources like wood or propane to provide cooking options on cloudy days.

Overall, a traditional solar cooker is primarily designed for sunny days.