The laws of physics are simple. There's a hard 500 nanometer limit on the size of objects that you can see through a conventional, optical microscope because that's the "diffraction limit" of a wavelength of visible light. Anything smaller than 500nm looks fuzzy and out of focus because it is simply too small for the optics to properly focus on. Basically diffraction limits exist because the light that bounces off the object and into your eyeballs is larger than the thing you're looking at. The problem is magnified when there are multiple sub-500 nm items in your field of view because their "fuzziness" overlaps and further obscures the view. But that's where super-resolution fluorescence microscopy comes in.