I think it has something to do with electronics regulations in the EU. There has been something about getting rid of chargers on mobile phones + unification of charger port (nearly all phones except for iPhones and some dumbphones use MicroUSB for charging now) to reduce electronic waste. Maybe the 3DS is also affected by this measure, as there's a lot of people that might already have the chargers from a DSi, DSiXL or 3DS around. For example, I already have 3 charger cables for my 3DS and didn't even have DSi back in the day: the one that came with the system, an USB cable that I got for travels and another USB one that came with an extra battery pack.