Because I already have several USB drives, and see no need to buy more for the purpose of using FireWire. The USB drives do exactly what I need them to do (work well between Macs and PCs). Why replace them?
No one ever advocated replacing them. I don't understand why a person wouldn't buy a dual interface drive in the first place. (assuming the difference in price to be negligible)
I mean, if you were to buy a new drive tomorrow would you buy a USB only drive or a dual interface USB/FW drive?