There are several reasons. One is the belief that spirits draw upon energy to manifest, so during the day the spirits can simply draw off the natural energies from the sun but by removing the sun (not literally) and limiting the power being used your essentially forcing the manifestation to draw upon your own energies or that of your equipment. Another reason is that cities/ towns are less active, ambient noise is minimal, which makes it easier to debunk/prove what noises are.
Why do you wear dark clothing on investigations?
The reason behind wearing dark colored clothing is because light doesn’t reflect off of it very well, and if you wear “loud” colors, you will stand out more on videos when we are reviewing for evidence.