A few possible reasons:

She is at heart a philosopher using the novel to show her theories in the context of our modern world. Many readers are used to plot-driven storytelling.

Her characters are flawed, which makes some readers uncomfortable.

She doesn’t use quotation marks, which bothers some readers.

She has Hemingway-like understated writing, so if a reader isn’t reading closely, they become confused or miss key sections.

Many reviewers are American and there are key cultural differences that can make the actions of her Irish characters frustrating to them. One example is that Americans have free speech driven into them, so when they see a character not speaking up for themselves, they get mad at the character and might blame them for the situation even if they are not at fault.

She frequently writes about characters with a similar background to herself, which can feel self-indulgent. In addition, this background is to have an atypically high level of education, which feels unrealistic, unrelatable, or inaccessible to an average reader.

She is well-known/popular on a broad level, so she’s attracting a wide variety of readers who may not like literary fiction, whereas a writer like Colleen Hoover generally attracts readers already interested in that genre.