Probably because almost every Spanish-speaking singer or singing group has done a cover of it, and it's practically a Spanish standard now. Many of the members here, myself included, were born after 1977, the year of Elvis Presley's death: how is it that we know Jailhouse Rock and Hound Dog? It becomes part of the culture. I heard several covers of Eres tú while still in Cuba, and my favourite was sung by Celia Cruz in concert until I heard the original recording by Mocedades. Today the Mocedades version remains my favourite. It's hard to improve on the original.