I think it's a matter of there being some sort of ceiling that's been hit when trying to read such tightly packed data on the platters. Before, the read speeds would increase as the density increased, but if there's a physics limit to how fast the data can be read/written, then just the size will increase and that's it.
It would be nice if drives would have gone up in speed as their size increased. A 6x increase in sustained read/write speeds would put these 12TB drives somewhere near 1200MB/s for peak read speeds (based on a peak of 200MB/s for the 2TB HGSTs I've got).