To me, this is like hearing "why were there never novels written about Neon Genesis Evangelion?"
NGE, like Frank's Dune novels, found a lot of it's fanbase because of it's unique approach to presenting itself; while Frank Herbert deconstructed the Science Fiction genre in the process of writing his books, Hideaki Anno deconstructed the Giant Fighting Robot genre of Anime while producing NGE. In my opinion, both are very well presented stories, but the fact of the matter is that they would not be as revered as they are now had it not been for their originality--Frank Herbert is a well respected writer, it is his writing in an age where simply more people read books that made the Dune novels so phenomenally well respected and famous. On the flipside, Anno is a well respected director, and it was primarily the visuals and music in NGE (the music, specifically, drawing credit from Shiro Sagisu) in an age where more people watched movies and TV shows that pulled me and many others into taking Anime seriously. If Dune was not a book series, and if NGE was not a TV show, we would not be having this conversation.
You see what I said there? The bit where I mentioned "watching anime seriously"? Before I watched NGE, I was a fan of the Dune novels; now, don't get me wrong, I found NGE to be really enjoyable, I even listen to the soundtrack sometimes, but I was only able to start watching because I had an open mind and because I believed a friend that told me it was not going to be silly. That's what I'm getting at. Dune is a serious story that Frank wrote to immortalize many of his philosophies in an ageless series of books; it is well respected because of his style of writing, his dialogue, and his ideas that, were they not written down, could be easily overlooked in other mediums of storytelling like music, movies, plays, or (you guessed it) TV shows.
What I'm saying is, Dune is a story that was meant from the very beginning to be told in books; even if (and I mean eeeeeven if) you could make a movie or TV show based on Dune that is good enough to receive a level of critical acclaim even close to the books, it would likely be considered to be, at the most, only "like Dune", and, were it in the form of Anime, simply not taken as seriously. It's not me, man--trust me, I've tried to convince at least a dozen of my friends that NGE is worth watching, but they are all convinced that it's some weeaboo bullshit and won't touch it. The Dune series began in 1965 and is still around today because of how well the books were written. If an Anime adaption were made right now, it would be:
Likely a ploy for Brian to make more money off of his father's ideas.
Likely not worth the effort, as it would appeal to only some book-only Dune fans, only some fans of both Dune and Anime, and only some new to both.
Likely laughed at by those who abstain from critical insight into cartoons.
It wouldn't be worth it, it would only appeal to some people, and it was never really meant to be something other than a book anyway.