Didn't they used to show double features way back in the day at the theater where the first one was really low budget and terrible? I imagine that was from the time when it was five cents for both movies lol. And then you had all those awful low budget things made in the Philippines at the drive ins. I imagine some people developed a certain fondness for those. Or just a fondness for the drive in experience.
I agree with the low budget, but I don't know whether I always agree with the terrible. Terrible sounds more like the C movie and especially the Z movie. As an example of how good a B movie could be is 1952's "The Narrow Margin" w/ Charles McGraw, Marie Windsor, and Jacqueline White, directed by Richard Fleischer, and with a screenplay by Earl Felton, who was nominated for an Oscar for his work. An RKO picutre, it is actually thought to be a better film than many of RKO's A movies.
Actually, the B movie apparently dates back to the early days of films, back to the 1920s, with the decline beginning in the 1980's. Though, for me, the decline began a decade earlier in the 1970's, when theaters, or the ones I attended, stopped showing 2 movies for the price of 1.
Another qualification of the B movie was its shorter length or from 1 hour to 90 minutes or often shorter than the main feature or the A movie shown with the B movie.