It takes longer to traverse a larger cache, so if you buy a smaller drive the partial assumption is you will be doing smaller file transfers in which a smaller cache is faster for cache hits. It also has to do with price point, smaller capacity, smaller profit, so using less ram helps keep the revenue higher.
Also note that currently ALL had drive manufactures are having issues with 2TB. The current best head/media combo (something like TDK/fujitsu)with a marvel channel is having difficulty in signal to noise ratios at 2TB densities. The yield for 4 platter 8 head drives is very low, making it not feasible to ship. Two head 1 disc on the other hand has much higher yields and that's why the 7200.12 is a one disc 2 head drive that is shipping, but a 4 disc model of the same density is not shipping. Until there is a break through in head/media/channel technology we will most likely not see large volumes on 2TB drives. The other options are, slower spin speed with higher over sample (WD uses this currently among other things) or moving to a larger sector size than 512 bytes to increase format efficiency.