Previous studies by various groups have found that time-to-completion rates for humanities fields lag those for others, and the Council of Graduate Schools effort provided more confirmation. In the physical and biological science and technology fields, more than half of those in entering cohorts are earning a doctorate between year six and seven of a program. In the social sciences, year seven sees only a completion rate of just over 40 percent; in the humanities the figure is 29 percent.
From this Inside HigherEd story that has a link to a recent report -- with data -- on PhD completion rates. The question of 'why it takes so long' is still unanswered.
Sometime ago, Sunil Laxman pondered this question. At that time, he asked me for my comment, but I have to confess that I really have no special insights to offer.