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Bitsquid supports many platforms, including next-gen consoles, so it's not the case
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The Bitsquid site lists Windows, PS4, PS3, X360, Shield, Android, and iOS.
Unity supports iOS, Android, Windows Phone 8, Windows, Mac, Linux, Xbox 360, PS3, Wii U, and (since NnT will be released on them we can also assume) Vita and PS4.
Unity is also generally more widely used in the industry, so probably better suits their workflow and allows for a them to more easily find talent with experience of their engine.