it's simply a matter of which image is brighter. the room's reflection is just as bright in daytime as at night, but that reflection is overwhelmed by sunlit objects outside in the daytime. an equivalent example with sound waves instead of light would explain why you can hear someone whispering in a quiet room, but have no chance in a room with your teenager's boom box playing. the daylight outside is like a boom box, overwhelming the whispered reflection of the room. when nature turns off the boom box at night, the whispered reflection is much easier to sense. 