According to early sales numbers from Nintendo, the digital version of New Super Mario Bros 2 managed to contribute to only about 5% of the game's total sales so far.
While the numbers are low, Nintendo states they are still happy with their experiment and still plan to provide more games having digital copies in the future for the 3DS.
A potential big reason for the lack of sales in Europe was due to GAME and a few retailers having discounts when the game was released which made the digital version more expensive but the same wasn't true in the US or Japan regions.
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