I come here not to bury Ulysses, but to praise it. [lest this thread turn into just another catalog of books we don’t like]
Let’s start with the notion that nobody likes it - on Goodreads, it has 42,852 five-star ratings, 30,019 four-star ratings, and an overall rating of 3.73…that rating is “okay”, but it’s clear that many people, like me, like it quite a bit.
As to why, I am only going to describe what I like about it, not speak in absolutes…people like different things in their reading.
What I like, then, is that for me Ulysses is a text rich in empathy, ambiguity, and technical complexity. I’ve read it maybe half a dozen times, and each time I come away with a different feeling for some of the characters, and I’ve also noticed more of the connections Joyce made in the text - Joyce was a great one for planting a seed in one chapter that gets echoed in a thematically-appropriate way in another chapter. Sometimes it’s direct, sometimes it’s just a hint of phrase that recurs as (for example) Bloom waits for the 4 o’clock hour when, he rightly suspects, his wife Molly will engage in a dalliance, or (for another example) how the memory of his dead son lingers at the edges as he tries to usher the young man Stephen Dedalus through some dicey situations.
Joyce also does a lot of showing-off in his text choices - some of which I enjoy greatly, some of which I can take or leave - and in his references to philosophers, literature, science, etc, which usually shine a light on something.
The book is a lot of work, and I enjoy that work, but I’ve never recommended it to anyone for that reason.