I really think it's because we hear them so often. It becomes branded in our minds, so we remember the acronyms instead of the lengthy name (or in the case of radio stations, their "name" is either the initials or something completely different). I think if Amazon started calling itself AMZ or an unrelated BCNW, most people wouldn't remember it.....and they'd have to completely re-brand themselves as those letters.
And best of luck finding that "perfect name" for yourself.
