The reason: Too much is published for anyone to read all that is important in their own areas of research interest, let alone all that is important in the field. As Markus Bockmuehl puts it, "the sheer flood of both printed and electronic publication has massively advanced the balkanization of a subject that any commonsense observer would regard as concerned with a fairly manageable source text--a mere 138,000 words" (Seeing the Word, 34). In a recent interview, on Justin Taylor's Between Two Worlds blog, Peter Williams commented: "It may be disheartening for PhD students to learn, but it is not particularly likely nowadays that many people will read their dissertation." I am not particularly hopeful about the fate of published dissertations either. Here's another comment by Chris VanLandingham, whose Ph.D. dissertation just came out with Hendrickson:
"When I sent the manuscript to Hendrickson four years ago, I had hoped that a book-length publication on my résumé would lead to an interview for a tenure-track position teaching in my field of early Judaism and Christian origins. By this time I had applied for over 250 such positions, but had not been invited to a single interview. I am still waiting for that first interview. So, my hopes are actually quite mundane. Otherwise, my book is one of 100,000 published this year, so in reality I don't expect my thesis to achieve anything of significance. Personally, though, it sure feels good getting one's thoughts down on paper." (The full interview can be found on Chris Tilling's blog, Chrisendom, here.)To cope with this onslaught of verbiage, people read what their friends write, or their friends' friends, or the friends of their teachers, or the friends of their teachers' friends. Sometimes they read what gets press in the blogosphere. (VanLandingham's book, Judgment and Justification in Early Judaism and the Apostle Paul, actually sounds interesting, so I ordered a copy. Whether I will actually get around to reading it is another question.)
If you have published something in a well-known series or journal, others scholars who work in your particular sub-specialty will hopefully skim what you have written--especially if it is well-written and the title describes the topic. But from time to time a reality check is in order. Some people are famous in a sub-specialty of five or twenty-five or five hundred. (Honestly, how many people are interested in the reception history of Malachi 3?) No non-academics care.
A very few are fortunate enough to write something important that gets recognized. And they, more often than not, are well-connected.
Update: More here.