Doctor of Arts candidates want to learn how to teach. While they produce large bodies of research-driven work, it is not the focus of their studies. Rather, they attend classes and engage in discussions about the actual business of teaching---what is effective, what is not effective and various teaching philosophies and strategies. This means they graduate equipped to be teachers who research rather than the converse.
Ph.D. candidates, on the other hand, are completely focused on their research. They take classes on their subject (chemistry or philosophy, for example) for a couple of years, but the vast majority of their studies (five years or more) are spent carrying out one large body of research, which they publish as a dissertation. This means they graduate as experts in their field but not necessarily experts in teaching. If they move on to academic careers when they graduate, they are researchers who teach rather than the converse.
Universities tend to hire people with Ph.D.s rather than D.A.s. This because a university is essentially a research institution that also teaches; research is what gives it its credibility and, more importantly, government and corporate funding. So, while a D.A. may be a better teacher than a Ph.D., it is the research that a university is ultimately concerned with, and therefore will be more inclined to hire someone with a strong research background rather than a strong teaching background.
While universities tend to prefer Ph.D. candidates, that is not to say that D.A. graduates are going to be unemployed. The major criticism leveled at D.A. graduates is that they do not have the same research background as Ph.D. graduates. However, they still have some research skills---most D.A. programs require their candidates to have research-based Master's degrees and to carry out a number of research projects while they are studying. So, while D.A. graduates are rare and the conventional wisdom is that a Ph.D. teaches research skills more effectively, both programs prepare graduates for a careers in academia.
However, as of January 2011, the Ph.D. graduates were the ones getting jobs---and even they were having a hard time, with many more graduates per year than available positions, particularly in the humanities.