Harder to adapt than LOTR or ASOIAF
I think the essential reason why no one has been able to really do Dune justice is because it may be the hardest work of SF/fantasy to adapt to the big or small screen, more difficult for instance than Lord of the Rings or A Song of Ice and Fire, the first of which of course succeeded amazingly and the second of which is (so far) doing the same.
Dune is so thematically complex and deep, the plotting so labyrinthine, the characters and setting so unusual, it surpasses both of the previous works, IMO. It would be best suited to a miniseries format - as it would require several movies to properly flesh out the story and characters, and the story is not structured for separate feature films.
The miniseries was an improvement on the 1984 film, but that's not saying much (hey, it's Sting in blue plastic underwear!). I hope they try again. It is possible, Children of Dune was quite good and shows that the story has potential for greatness onscreen.