There’s several factors here.

The most important probably being the energy per square meter. Higher latitudes gets less energy per square meter than equatorial latitudes.

There’s also the mentioned cloud cover and atmosphere density.

The climate it’s also important. As higher latitudes tends to be more cloudy.

And sun hours, here is not about the total energy but how it’s distributed. As sun hours are more estable near the equator (12 hours light 12 hours dark) while in higher latitudes you can get 4 hours light some times of the year that can’t amount to nothing, and 20 hours of day other times of the year which are nice, but there’s no way to store that energy for the winter lack of sunlight.