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During TGS 2015, Japanese gaming site 4Gamer.net published an interview with the producer of the game in which he shares many details including why Hyrule Warriors Legends will feature stereoscopic 3D on New 3DS models only.
In his explanation of the development of Hyrule Warriors Legends for the 3DS, Yosuke Hayashi, the game’s producer, said one of the challenges for the portable version was accommodating the game to show “the right amount of enemies on screen.” In order to make the game work as desired by the producer, Mr. Hayashi decided to disable the stereoscopic 3D feature on regular 3DS models. Owners of the New 3DS models will still be able to experience the game in 3D due to the portable’s more powerful CPU.
The absence of the 3D feature in this action-packed Zelda game will be unfortunate for fans who enjoy it on the 3DS, still, the game will include amiibo compatibility and DLC characters (as in the Wii U version).
[Source]: 4Gamer.net: ［TGS 2015］「ゼルダ無双 ハイラルオールスターズ」プロデューサーインタビュー。amiiboやすれちがい通信にも対応した長く遊べる作品に.
[Via]: Perfectly Nintendo: [TGS 15] Hyrule Warriors Legends: more characters to be revealed, amiibo support confirmed.