"Seeing" (and for that matter any sensor deployed to detect a signal) must have a contrast. 'Contrast' puts a signal 'stand' against a low-contrast background. It is the cardinal rule of any 'Instrumentation' technique.
The lower limit is set by "noise". If the ambient noise level is low even a weak (or faint) signal will stand out against a low-noise backdrop. But if the noise level (we are dealing with energies here) is relatively high, signal needs to be higher than it, to be noticeable. In other words a weak signal gets drowned (or buried) in noise.
During night the atmosphere has less light to scatter spreading nothing (like light). In day, the overpowering Sunlight gets scattered uniformly raising the ambient (noise) level high failing to bring out the contrast of faint object. City lights and its excessive street lighting bestows it day-light ambiance, similar to day where we don't see any stars. On Moon where there is no atmosphere worth the name to scatter Sunlight, even in a Lunar day, the sky (other than the Sun's disc) looks pitch-dark with stars standing out (I was told). For good viewing from Earth, it is necessary that the city lights should be shunned that cause 'light pollution'.
'Light pollution' can be defined in terms of ambient 'light' (energy) that sets the threshold noise above which an object like the star Sirius can be observed. Or it can be set in 'energy' (or 'power' which is energy per unit time) units.