Kafka is only slightly less popular than Proust -- which I find surprising, since Proust would seem to have much greater snob appeal, Kafka being the emo band whose posters you plastered your walls with in high school, but who you loudly deny ever having liked once you're a grown-up. Unfortunately I can't easily tell where these articles are coming from -- are the upper crust of arts departments writing mostly about Proust and Joyce, while the reject departments with no friends write mostly about Kafka and Salinger? I have no intuition here, so arts people, feel free to weigh in.
At any rate, we see that, just as with their theoretical badges, academics make their consumption a fashion symbol too. Between 1935 and 1945, the three Modernists begin to soar in popularity, but somewhere between 1955 and 1965 they hit diminishing returns, peak around 1975, and get tossed out after that. Note that this is not due to the rise of Postmodernism -- that only got started in the mid-1970s and was big in the 1980s and '90s. Already by 1965, Modernist authors saw their growth slow down. Besides, Postmodernism was attacking the assumptions of another group of academics, rather than attacking a group of authors, painters, or musicians.
The data only go up through 2001. Just eyeballing it, it's conceivable that by 2025, these three Modernists won't be given more respect than established authors like Jane Austen, and of course some may see their popularity plummet further to zero. This is a separate question from their artistic merit, obviously.
The mania for categorizing authors by time period and not content continues, but a good point is made: by 1965, modernism had been replaced by radical progressivism/liberalism as a revolutionary norm, and from it postmodernism was birthed. Modernists were cast aside because they didn't belong to the new movement, but more fundamentally, because their theories didn't explain how the world had developed since their works were written.