I personally enjoy the night versions of each map slightly more than the day maps. If I had to guess, perhaps the night versions are not available in Private Play due to an oversight by the developers. If I had the option to select between the day and night version in Private Play, I would pick night most of the time. It would be cool to see day versions for the Zomburbia and Zombopolis maps as I believe you can only ever play them at Night. It makes sense to have a night theme for Moon Base Z though as you are quite literally in space so I don't see a day version making sense for that maps.
