Why are most of 2TB hard disks released 5400 RPM?

It's not just a simple matter of putting a faster motor in. I think that because 2TB drives have an even higher areal density, more advanced silicon is needed to process the data at such high speeds. Also, with the increased areal density, data is physically being read faster anyway.

It looks like the 7,200rpm drives have 4 or 5 platters and were made with older technology. The new 3 platter 2TB drives step up the areal density, and slow the spindle speed. As the silicon technology advances, they will speed up to 7,200rpm again, and then add more platters to increase storage, and then increase areal density and slow down the spindles, and so on...

Concerning data transfer, generally 7200 RPM drives are slightly faster; but the speed is more dependent on the other factors, i.e. sequential read, random read, sequential write, and random write speeds. Sequential read is typically the most important. Random read and sequential write are usually the second-most important.

Another important thing is the lower RPM drives last longer, use less battery power and run cooler than the higher RPM drives.