[_dps]

Two likely contributors:

1) due to population effects, academic positions are much more competitive now than they were in say 1970; if you figure that the top 50 research universities are not generally expanding the number of professors, and that new professors generally also come from those top 50 research universities, then on average a top-50-research-university professor will generate one new such professor in a career, despite having 10-100x as many graduate students (this was different in the 70s when the university system was rapidly expanding).

2) the increasing desire for fairness in hiring and promotion (by itself, a good thing) means that you need to be able to resolve hiring and promotion disputes with something both objective and external to the university (in the same way some undergraduate institutions put more admissions weight on external and objective metrics like standardized tests compared to more easily game-able internal metrics like high school class grades)