One theory...bigs are not central to the oscar Weber system, therefore he doesn't expend as much recruiting capital on a resource that is already overvalued because of limits to market supply? In general, current era Nova excluded, the blue bloods get the quality bigs, with certain programs in which the big is central to the system getting in on some as well (Huggins comes to mind).
Another theory, there are less quality bigs coming out of high school than the past as a result of nobody being trained to play with their back to the basket, at least relative to the way it used to be?