I love his writing, but I don't know why he is considered one of the great fiction writers in English. Why was James Joyce's writing so "revolutionary" for its time? I know that at one time originally it was considered risque for some reason, but I could ask the same thing about Ernest Hemingway.
Joyce's work was revolutionary in a number of ways. In most of them it was not that he innovated new techniques, but in the degree to which he pushed them.
To start with, it is worth considering what the novel traditionally looked like at the end of the 19th century. Victorian literature was almost entirely written as a linear narrative with an omniscient narrator. All the great authors of the era - Dickens, Bronte, Hardy - wrote in this style. This is sometimes known as realism: the attempt to paint a literary picture of belivable characters in the real world.
Joyce, by contrast, was more interested in literature as an artistic technique. He used metaphor, motifs and symbolism far more heavily, leading to works that - like a painting - invited multiple interpretations. These were not new tools to authors, of course: Joyce's innovation was the extent to which he deployed them. Indeed as his career went on, traditional structures like plot and character began to play second fiddle to artistic motifs. You can see this in the second half of Ulysses and, at its most extreme, in the nonlinear word soup of Finnegans Wake which defies any traditional approach to reading.
In this, he aligned himself with modernist painters like Picasso. The novel was still a relatively new form compared with other kinds of art, and many critics saw it as "low" populist art, for mass public consumption. Joyce was pushing its boundaries into alignment with avant-garde art in other fields. Arguably we thus have him to thank for the damaging split between popular and highbrow literature that persists to this day.
Stream of Conciousness was also not new - I vaguley remember reading something very much like it Jane Austen's Emma. But again, his approach to it was entirely new. Rather than writing it in the traditional "omniscient" point of view, he let it flow freely, often into near-nonsense, just as thoughts and ideas sometimes blindside us through the day. Arguably this was the death-knell of the first-person narration that had been dominant up to this point. Also, arguably, it was more "realistic" than the realism of preceeding novels.
This lead to the portrayl of inner thoughts and feelings which broke taboos in subject matter. Yet again, Joyce was not the first here, but he pushed the envelope much further than other authors had done. His frank protrayals of sex and defecation are well known (his letters to his wife reveal he was a something of a scat fetishist himself). But it's also worth remembering that he was unusually blunt when it came to politics, particularly the troubled politics of his native Ireland, which were not considered a fit subject for fiction at the time.
Other literary figures of his era, such as Hemingway, remained constrained by the traditional realistic approach to the novel. Joyce was - and to some extent remains - unique. I'm not aware of anything else like Finnegan's Wake. He recognised that an author's style of writing is a constraint on what they wrote about. Prior authors had seen this as a regrettable limitation but Joyce embraced it: by jettisoning style, he left himself free to portray the world as he saw fit.
The questioner loves James Joyce's writing but wonders why his work is considered "revolutionary". This question is not so easy to answer in a short Stack Exchange post, and many books have been written about James Joyce.
James Joyce advances avant-garde literary techniques that began with Laurence Sterne's The Life and Opinions of Tristram Shandy, Gentleman.
James Joyce as a fiction writer pioneered several literary techniques, most notably the stream of consciousness technique where the narrator's thoughts wander, and are related to the reader without a rational commentary. The effect creates a deep level of ambiguity in what the character is referring to. Even a sense of time and place may be obscured.
James Joyce pioneered a technique of playing with language, substituting, in some cases, words that are etymologically related or are homophones of the token word. This makes James Joyce's writing notoriously difficult and often readers of Ulysses use some sort of companion to assist in understanding the text. This should not daunt the reader who wants to read Joyce as Joyce himself wanted to be understood by the Common Reader. To demonstrate how this is possible, a short quotation from Finnegans Wake, James Joyce's most difficult work, will be quoted without commentary. An alert reader will notice that the passage is controversial even today, particularly with very staunch conservatives who are inflexible, intolerant and have religious views that border on totalitarian thinking:
"In the name of Annah the Allmazifal, the Everliving, the Bringer
of Plurabilities, haloed be her eve, her singtime sung, her rill be
run, unhemmed as it is uneven!"
James Joyce pushed the formal boundaries of the novel to the ultimate limit. His techniques and style influenced other pioneering writers such as the nobel prize winning William Faulkner, and Virginia Woolf who acknowledged James Joyce to be the better writer. One can recognize inflections of Joyce's work in nobel laureate Ari Zimmerman's first novel. Zimmerman cites William Burroughs as an influence, whose literary experimentations owe a debt to James Joyce. Nobel laureate, Saul Bellow, also plays with narrative techniques that were invented by James Joyce.