Some articles are banned from the game if they're not in a game's theme or if they have too few views -- these are defined in each game's settings!
Categories are a way to play the game with a specific theme or set of articles.
I'm working to implement a way to view and edit the lists in game, but for now, you can find these lists here :)
Currently the Default Banlist won't allow you buy the Wikipedia articles for specific days, months, or annual holidays (e.g. May 15, February, Christmas).