It comes from how movies used to be shown in theaters. The double feature was much more common and between movies there would be cartoons, short films, and news reels. So one ticket would get you a theater experience all day pretty much. Pairing movies usually meant one higher profile picture and one with a smaller budget. The second smaller film is the B movie. Smaller budgets meant more genre pictures with lesser known actors. So you get cheap sci-fi and monster movies mostly but the term B movie just refers to its lesser standing. It’s like the opposite of Oscar bait.