Data density is still increasing -- look at 2.5in drives, which have carried on getting bigger -- but the manufacturers have for some reason been using that headroom to build drives the same capacity with fewer platters, rather than larger capacity. Presumably part of the reason is the thing with traditional partition tables only dealing with drives up to 2TB, but that can't be the whole story (people have had RAID arrays >2TB usable for a while), and you'd still think there'd be a market for capacious (if slow) drives for media servers and so on.

Peter