What's holding it back? Nothing really, the basic drive can be engineered to work with either the SCSI or EIDE interface.
Then why isn't it here? Because only tech-heads like us want it.
The majority of computer users are still getting by just fine
with 5400rpm drives, and the advances in areal density and
reliability on those make them come out in faster models than
were available a few years ago. Its cheaper and easier to mass-produce and sell 5400rpm drives than 7200rpm. When the need
for 7200rpm drives becomes the baseline, then we will start to
see these companies concentrate on 10000rpm for the high end.
Right now, they are more concerned with capacity than speed.
Hard drives may seem slow compared to CPU, memory, and video, but
they are still the fastest mass storage devices connected to the
system. As the demand for ATA-100/Ultra160 devices grows, then
you will start to see the drive makers concentrate on pushing
the throughput more.