I have thought about this for a long time. It is one of the reasons why I started this blog - to raise awareness about the Nag Hammadi writings and to focus attention on why they are so vital for us to study as biblical scholars. I'm also trying to get an exhibit of Nag Hammadi manuscripts to accompany the Codex Judas Congress, but this will depend on whether or not the Egyptian Supreme Council of Antiquities will allow them to leave the Coptic Museum. Let us hope!
I think that scholars of early Christianity are more interested in the Dead Sea Scrolls for these simple reasons:
1. The Nag Hammadi documents got labeled pejoratively "Gnostic" from the beginning. Doresse when he looked at the first codex and read a few lines made the announcement that these books were Gnostic writings. In the 1940s and 1950s this was not a positive spin (and frankly it still isn't). In fact, many early commentators talk about these texts as "perverting" scripture and the "real" Christian faith. Since our field is so dominated by this theological perspective, the study of "perverted" literature was not considered important. Many scholars (even yet today) wonder why any "real" biblical scholar would want to waste his or her time studying perverted Christianity. I know this because scholars have said this directly to me, calling the literature "crazy" and "a waste of time."
The Dead Sea Scrolls never had this labeling problem. My understanding of the spin originally put on the scrolls is that it was not one of a perversion of "real" Judaism, but of a disgruntled sect of Judaism (the Essenes), a sect that might tell us something about Jesus and early Christianity. For scholars of Christianity at the time, this was a positive thing because it helped them explain the formation of Christianity which was for them like the Essene movement, a critique or revolt against Judaism. From what I can tell, this was part of the anti-Semitic explanation of Christian Origins common at the time. The other explanation was to erase its Jewish roots by demonstrating the victory of Hellenistic thought and practices. So Christianity was understood to be a Gentile religion that superceded and erased the Jewish one.
2. Because the Nag Hammadi materials are in Coptic, they are difficult for the majority of scholars to assess in the original language. This is not the case with the Dead Sea Scrolls whose original language is much easier for scholars of the bible to handle.
3. The Nag Hammadi materials, for the most part, are from the second and third centuries. Most biblical scholars don't even study the ante-Nicene fathers let alone the Nag Hammadi documents because they perceive the time period to be later than the NT, so therefore inconsequential to biblical studies. This is not so for the Dead Sea Scrolls which predate the NT writings.
4. Gnosticism is a word with a lot of baggage, most of it completely inaccurate. One of these inaccuracies is the belief that Gnosticism is a religion in antiquity that is separate from Judaism and Christianity, that is is a revolt against Judaism and Christianity. So scholars who understand Gnosticism as a religion separate from/revolting against Judaism and Christianity, do not see that the study of Gnosticism has anything to contribute to the study of the bible or early Christianity.
In future posts, I will address why each of these assumptions needs to be reassessed by all of us studying early Christianity.