What is one reason for the variations of sunlight in different places on earth?

The Earth's tilted axis of rotation combined with the Earth's movement around the Sun causes the seasons.

The Southern Hemisphere is "tilted" towards the Sun, then the Northern hemisphere, in a yearly cycle.

In January it's summer in the southern hemisphere and winter in the northern hemisphere.

Then, about six months later, it's summer in the northern hemisphere.

The effect of being tilted towards the Sun is to extend the hours of daylight and to increase the height of the Sun above the horizon.

Both those effects cause an increase in solar energy received.

So, you can see that the tilt of the Earth's axis is the basic cause of the seasonal variations in energy received at any one place.