You know how they say optical microscopes are limited by the wavelength of visible light, because they don't really interact with things smaller than the wavelengths used? Well, it's apparently related to that. To get better resolution in viewing something, you need a smaller wavelength; but shorter wavelength of a photon (or other wave) means increasing its energy. Whenever a photon hits a particle or anything else, it imparts some energy to it, thereby altering the observed particle or system. So the higher the resolution of imaging the particle, the more accurate you know its position. But at the same time, you're imparting more energy to it by decreasing the photon's wavelength, so you don't know the particle's momentum as accurately anymore.
