It's very short and the prose is straightforward. Most "alien" words are real-world words with alternative spellings. There is a glossary, and in any case Lord of the Rings language (English and made up) is way more difficult. The plot of A Game of Thrones is likewise harder to follow than Dune's, with more factions, more intrigue, and mysteries that are still unfolding many books within the series.
There is political intrigue in Dune ("plots within plots within plots") but it's often explained out loud or with inner monologue by the main characters. The Baron Harkonnen explains very early his plot; a deeper conspiracy is soon explained by other characters. The religion and manipulation of the Bene Gesserit is explained very clearly by Jessica. There are almost no "long" mysteries that are not explained close to their introduction.
The book also wears its real-world influences on its sleeves.
So the plot is not particularly convoluted, the prose is straightforward and the made up words have real equivalents (mostly).
So why do people find it difficult?