I think it's more of a crew based decision (but a cautious one at that). Due to the nature of many travelers who jobs keep them on a plane more times than not, it's always 'night' somewhere and you're going to have a passenger who's jetlagged or simply wants sleep.

This crew-based decision comes from the fact that many times I've seen a person ask the FA to ask another passenger a couple rows ahead/back to lower the window shade during a daytime flight. You probably have more people asking for the shades to be closed, rather than to be open. Therefore, crews err on the cautious side.