User blog comment:Master Sima Yi/Assassinews 05-08-'14 – Assassin's Creed: Rogue announced/@comment-3446336-20140805220942/@comment-13471981-20140806175051

That was retconned in AC4; every human possesses the potential for Eagle Vision, but most people never unlock it. There's a rare few to whom it comes completely naturally (e.g our protagonists), while for others, it takes training (e.g Gavin Banks; apparently the use of Eagle Vision is exhausting for him).