Because in any given milli second, the amount of photons entering your eye from the sky illuminated by scattered light from the sun, is vastly larger than the number of photons your eye receives from faint stars by of the order of thousands to millions to one. Its purely a matter of contrast, and although the photons from every star in the night sky are still getting through to the ground, the sky background 'irradiance' in photons per square arcminute per square millimeter of pupil aperture is so large that the eye does not have the acuity or dynamic range to detect both the star and the nearby sky. Also, because of the bright sky illumination, the pupil of the eye 'stops down' to nearly its minimum diameter which makes it far less capable of detecting faint structure in the sky.