The thing is that, since each user map has a completely different theme (except for the more generic ones), choosing e.g. a city background would be unsuitable for a jungle themed map. Thus, it needs to be a more generic background.
I could allow each player to upload their own backgrounds for their maps, but this would create inconsistent aesthetics throughout the game (and you can't trust the players to always upload good quality images).
I will put some thought into what would make a better background. Until deciding, I find it best to play safe and keep the generic skybox. Please endure it :)