Hey Rick!
That is a good question Eccman! I have to pretty much agree with what the others have said but wanted to add to that. As far as investigations go with commercial properties, most of the places that I've seen investigated are still open to the public and functioning during the day, which would make it very difficult to investigate at that time.
While, this isn't the only reason to investigate in the evening hours, I'm sure this plays a big roll when investigating certain places.
I have to agree too though, it does set more of a mood to watch an investigation going on at night rather then in broad day light.