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B movies are so called because they were the "B" movie at the cinema. You had an "A" film, which was the film you'd usually gone to see and had the big stars in them (hence the A-List), essentially, the one you paid for but before that there'd be a "B" picture. You'd usually get newsreels too and, if it was a childrens film, a cartoon or serial. So the "B" movie was a cheaper, 'lesser' film (but not arthouse or documentary.) Horror, Sci-Fi and Westerns were the staple of these. In the 50's, as a way of trying to get the tv audience and offer something a little more salacious than they could get away with elsewhere (which you could in the "B" films, as they were usually ignored by critics or the cinema papers.) These also appealled to the new teenage market, who were starting to have their own money and wanting their own things, hence the Double Feature. This was/is more of an American thing, at least, it was from a UK perspective.
This really blossomed in the 60's, where the social changes and ever reducing ticket sales meant that this growing market was played to more and more. I suppose the 60's was the breakout decade for these films, starting with Psycho and ending with Easy Rider, a new Hollywood was born and, in turn, a new independent cinema. However, it was still "B" movie fodder and, while there are early examples of things changing it's not until Jaws, Star Wars, Superman, Raiders Of The Lost Ark and, finally, E.T. that "B" movies were looked upon as "A" movies.
So horror, Sci-Fi and Childrens films (including Superhero/Fantasy) were/are all "B" movies, as with few rare exceptions, that's where you'd have found them. In the "B" movies.