The study of openings is a very easy thing to 'productise' in comparison to other elements of chess. One's progress with learning openings is particularly tangible and easy to measure, but the law of diminishing returns sets in pretty quickly. There is also the more practical element that openings have names; it's quite difficult as a beginner to talk about more philosophical or abstract parts of the game, such as identifying and referring to the situations when a certain bishop is strong or not.
In many ways, the focus on openings is not a new phenomenon - chess opening books always seem to have dominated the genre of chess books. They're also excellent ways for publishers to sell updated editions... New openings are discovered like clockwork; the genre exists alongside sport almanacs and travel timetables as a perfect way to keep the printing presses going!