Exactly!
What would D&D bring to the media that Lord of the Rings, Harry Potter, and Percy Jackson hasn't already?
Yep!
Lord of the Rings works because it was the original thing that kicked off the high-fantasy craze many decades ago. As such, it carries a certain cachet within the public imagination. People who have never picked up a Tolkien book know that Gandalf was a wizard and Bilbo was a hobbit. It has strong name recognition.
Harry Potter was derivative in it's initial concept (from Gaiman's Books of Magic), but went off in it's own direction. Most of the public weren't familiar with the Books of Magic, so Harry Potter looked like something new and original. It was one of the first children's series to develop a very strong adult following, and the author was very savvy about marketing and many other things. It is a very strong and valuable brand.
Percy Jackson took a very old concept that was not frequently used and brought it into the modern world.
Indiana Jones took a genre that had fallen out of favor but was still floating in the public consciousness and presented it in the dominant entertainment medium of the time, with just the right blend of things that could appeal to all ages.
Those are all strong brands that have been marketed well.
D&D doesn't have a strong brand in the public conscience, beyond an association with geeky teenagers and (for a few) the old urban legends about Satanism and suicide. The setting and concepts look cliche in today's world, and were cliche' back when it was first invented. What caught people's imagination in the early days of D&D was the idea of combining roleplaying and wargaming. It was highly innovative in the sense of establishing a new hobby and industry, but not in the sense of establishing a compelling fictional world. That doesn't translate well into brand marketing outside of the initial hobby community.
WotC and Hasbro don't seem to have a good grasp of Marketing and Brand Management 101. Though Hasbro is a big, old company, their experience is with board games and more traditional types of toys, rather than larger fictional world settings. WotC was a strong, innovative company years ago, but have really missed the mark in the last decade or so, at least when it comes to rpgs. I don't know if that came about from changes in personnel, growing pains, or what, but it has been pretty evident. They were smart to try to bring in long-established successful rpg designers like Monte Cook, but we all saw how that turned out.