I wonder, too, if it’s because of a lack of consensus about what certain accents connote. Pretty much all speakers of English share similar associations with a range of accents that feature frequently in film and TV: a posh English accent (RP) suggests class, erudition, or villainy; a New York accent is direct and no-nonsense; a broad Australian accent comes across as friendly and unpretentious, etc. Unless I’m mistaken, such a situation does not map onto Spanish. Speakers of Latin American varieties do not, for example, regard the Castilian variety as prestigious, so there is no obvious candidate for what would serve as the “posh” accent in a single Spanish dub. (
@Sir_Cliff, please correct me if I’m wrong.)