Are they doing something wrong? Or is there an actual appeal they're just not seeing? To me, the shrinking hero's appeal is the same those one-offs are tapping into - the chance to look at our world from a different vantage point, a perspective and scale that makes it seem strange and new. Sure, it's hard to see that when a lot of the available imagery has a hero caber tossing pencils, but shrink down even more and they're standing on top of dust particles, like Saint-Exupéry's Little Prince, or visiting electron-sized planets. These stories can have the thrill of looking through a microscope, or seeing pictures of the micro-world that's right under your nose every day, if only you could see it. But many balk at the notion that a superhero could be menaced by a fire ant or an alley cat, and even critically successful Atom and Ant-Man series in the past few years have played up the comedy and quirkiness of the concept.
Another reason to like the shrinking hero (or even the simply diminutive hero like the Golden Age Atom who was an unpowered brawler standing a mere 5'1"; now Al Pratt is another Giant-Man, sigh), is that they are natural underdogs. And who doesn't like an underdog? Young readers are likely smaller than the world around them, smaller than their bullies, too short to reach those high shelves and their parents vices and secrets. The shrinking hero deals with that all the time and prevails. One should feel a certain elation at their exploits, and at the larger bully's humiliation when he loses to "the little guy". It's even easy to play at shrinking heroes, since their usual fighting size is that of an action figure. I used to explore my house and back yard from that POV all the time.
But perhaps the kids today have been taught that bigger is better and no one cares about the diminutive hero. Or maybe the trope just needs a boost, like Edgar Wright's Ant-Man movie. Even so, I fear the Mighty Mites will never truly get the respect they deserve, always be filed in the silly column, never be cool badasses. But I'll keep pulling for them.