In my field (PhD in art history), I’ve observed a number of students who get to the dissertation stage and just get stalled.
There are a number of reasons why this occurs, some of which have been mentioned here already.
Typically, by the time an art history student starts their dissertation, they’ve spent the last four or five years doing coursework. 2-3 years at the MA level, then an additional 2 years of coursework as a doctoral student (usually completing the MA degree separate from a PhD program, but there are some schools which streamline the MA stage into the PhD program). After finishing coursework, you take your qualifying exams, the preparation for which might require a semester or so unto itself, during which time you get as deeply acquainted as possible with the scholarship in your major and (at least in my case) minor fields of specialization.
After the exams, you’re a doctoral candidate. Huzzah! Time to write the dissertation.
This is the point where many students fizzle out. For one thing, you’re no longer bound up with the structure of attending classes and preparing seminar papers that are due at the end of a term. Without the atmosphere of discipline and pressure that come with regular courses, many students just find it difficult to focus their attention on writing a book-length manuscript of original research–the final date is too far removed in the future to pile on the kind of immediate pressure of a deadline that you’re accustomed to as a grad student, and–let’s face it–grad students can procrastinate just as badly as undergrads (actually, this could be said about a large number of academics in general. But I digress).
On top of that, there’s the funding issue, already mentioned by some posters above. The best teaching gig you can hope for (and in art history, academia is pretty much the only arena your PhD is geared towards) is adjunct professor. It used to be possible to land a tenure-track assistant professor position as an ABD, with the expectation that you’d finish up the dissertation in your first couple of years as professor, but the reality today is that there are already so many art historians on the market with their PhD already in hand, most schools won’t bother with hiring ABDs.
The pay for adjunct positions is miserable, around $1000-$2000 per course taught per semester. 16 weeks on $2000 (and that’s really above average where I live) just isn’t possible to live on. So, you’d have to teach several courses just to make enough to get by on. The alternative is to find a regular 9-5 job, which would pay more than adjuncting, and since you’re not taking classes any longer, there’s no schedule conflict. But in either of these scenarios, you find most of your time is consumed by course preparation/grading or 8 hours of mindless drudgery, and by the end of the day, you’re pooped and in no frame of mind to write a dissertation chapter. Plus, there’s rarely a sense of urgency in finishing it quickly and getting it published right away, since the material is most likely so incredibly specialized that it’s unlikely that you have too much competition from other scholars.
Add into that mix a family or a relationship, and it’s easy to see how a PhD candidate can get completely sidetracked by non-academic concerns (the “real world” as it’s commonly known), or just generally unmotivated, and ultimately disengaged from the dissertation.
I feel lucky, because although I went through some of these challenges myself (including a stressful and ultimately doomed relationship), I occasionally got some extra funding through scholarships that helped me travel abroad to consult some of the archives and paintings that I was researching. Without those opportunities, I doubt I could have afforded those trips, and without those trips, the writing of the dissertation would have been further delayed. From the beginning of my candidacy until the defense of the finished dissertation, the process took about five years.
And then, I got even luckier and got hired for a tenure-track job less than one year after finishing the doctorate. But the job market for art historians is awful–far more candidates with the qualifications than there are available positions–and this surely is an additional factor that can discourage and de-motivate a PhD candidate while writing the dissertation.
One might try to spin that into a positive and say that all of this is a good thing, because the competitive nature of the field will ensure that only those with the right drive and commitment can survive, and the fewer PhDs on an already glutted job market, the better for everyone. On the other hand, there are a number of brilliant ABDs and PhDs who just get frustrated with the process and find work in another field, and the discipline certainly loses something in their absence.