Why is wavelength the main limiting factor on limit of resolution in light microscopy?

Wavelength is the limiting factor of resolution in light microscopy because you can't resolve the image of two particles (ie, the space between them) if it is smaller than the wavelength.

If you get overlap of wavelength, then, it isn't resolved because you'll get mixed wavelengths. This can be fixed by changing from light wavelength to electron wavelengths, and this is when electron microscopy becomes important for high resolution and high power magnification work.