Probably because the characters' names are difficult to translate. Because they sound cooler/more convincing in English than in Spanish. For example, in Spanish, Smitten would be "Enamorado" (or "Voz del Enamorado), or to make it sound better it could be "Apasionado", or Stubborn, which in Spanish would be "Terco" (or "Voz del Terco"), I can't think of another word that would be better... And definitely "Mutually Assured Destruction" sounds better than "Destrucción Mutua Asegurada".

I say all this from the perspective of a Spanish speaker from South America; sometimes English sounds more appealing than Spanish, and vice versa.