In Europe, you're only able to purchase mature Wii U eShop content during certain late-night hours. Not exactly the most user-friendly way to get things done. This is all because of German law, and Nintendo of Europe's headquarters happens to be in Germany.
Good news is, Nintendo is working on a fix. According to the latest ONM podcast, Nintendo is working to fix this issue, but no specific details were given. Let's hope this is cleared up early in 2013. Thanks to Axis1500 or the heads up!