In bright daylight, each scene holds a lot of light energy. Therefore, a well-illuminated scene does not need to be exposed through the lens element and onto the sensor for a very long time to achieve the required exposure value.
In cloudy weather, each scene holds less light. Therefore, the scene needs to be exposed through the lens element and onto the sensor for a longer period of time. An alternative to achieve the correct exposure value is to increase the iris opening, allowing more light to flow into the camera per unit of time.