why do you need more illumination on your microscope when using higher levels of magnification?

Answer 1

Because you are looking at a smaller area of the object and are physically closer to it. So you need more light, and in the right direction ti illuminate the object for comfortable viewing.

Answer 2

Given constant illumination flux over a given area, if you reduce the area observed (i.e. greater magnification), you need to increase the illumination to keep the intensity constant.

Answer 3

The field of vision is smaller and gathers less light.
You must increase the surface light intensity to maintain the same brightness.