The New Historians began their work because there was, in short, a wealth of new information available. Morris articulates it rather well, having been one of the people who was first on the ball in using the new information to craft a refined narrative of the state's history in its earliest years.

That information, as a quick aside, led him to publish both Birth and Birth Revisited, when even more information (as well as some claims that he was misinterpreted in the first book) became available, though that information was subsequently taken "off the market" not long after he read it, so it's not available anymore.

Basically, the New Historians benefited from the declassification and liberalization of the Israeli State Archives, including political and military documents alike. In 1982, the Israeli State Archive began to open its documents in larger numbers than before, and allowed the publication of documents that were certainly of an "embarrassing" nature, though it wasn't entirely happy to do so. This posed a conundrum for some, like Morris, who knew they could write a powerful and important study on Israeli history, but also knew that the story was not fully complete. After all, documents did remain classified, and there was only one side of the story; embarrassing and triumphant Israeli facts might be released in greater numbers than ever, but the Arab archives (to the extent they existed and were accurate anymore) remained closed. Nevertheless, historians work with what they have, and each brought to the table a different approach; Morris, for example, relied almost entirely on what documents he could find in the Israeli archives, eschewing what he called unreliable oral histories, and using British and American intelligence sources as well as the UN archives to supplement the few Arab documents he could find. Morris, contrary to assertions by some like Pappe, did state that he used Arab diaries and written experiences, but to round out the picture, saying that they didn't change any of his actual conclusions and were instead valuable for corroborating details. Others, like Pappe, were completely fine with using those oral histories to construct a narrative they thought was more compelling and/or accurate. While some Arab documents have been spirited away or viewed by historians, they're not exactly common or easily accessed.

So following that liberalization in 1982, the New Historians could suddenly conduct research through a mountain of information, resulting in the publication of these books and articles over the following years. Flapan was actually one of the first to "hit it big" with these discussions, as was Morris.

Of course, that's not where the story ends. The New Historians did make the biggest splash with their discoveries in the 1980s, but Morris writes in Birth Revisited that the archives were liberalized further in the 1990s, with declassification of Israeli Cabinet protocols, and hundreds of thousands of documents in the IDF and Haganah archives that were pretty much entirely closed prior to then. Yet some documents, viewed by him and a few others (like Avi Shlaim and Tom Segev) were reclassified following release, due to claims that the release was a mistake. It's undoubtedly clear that some documents were overlooked in batches that the state may have wanted to keep classified, as happens in any large-scale opening. But it's important to keep in mind, because what a historian could see on any given day might've changed.

If you're looking for the historical factors that changed it, those are it. Morris actually got a very early look in 1982 at documents that were then reclassified (or rather, his privileges were revoked), back when he was more of a journalist than a historian. Of course, political factors that may have made the situation more salient as the rewriting was happening could relate to the First Intifada, which was in full swing by the point of the New Historians' ascent, and which made their work significantly more relevant to the question of Palestinian discontent and the Intifada that began in 1988, just a year after Flapan and Morris both published books.

Re-uploaded my question to correct grammar in title and to list 2 books I've read so far.

Thanks