I think it's because that's when people have free time to be at the computer... I love staring at my tank under the moonlights. Also tend to be here till 3 am most nights... Haha
The best time to look at your tanks is when the moonlights are on!
I agree with nate82 its because people have time to check out the forums at night.
If you don't already have a moonlight I recommend picking one up the shimmer and make your tank look great.
WOW, all of my corals were glowing nuclear green, or orange, or red, the colors were amazing! My radioactive dragon eyes glowed like a nuclear fueal rod! (lol)