I suspect that it is largely due to the fact that transfer rates with USB are usually slower and less reliable. Also, the performance of the USB devices degrades as you connect more and more USB devices to one USB hub.
You might consider looking into a USB alternative like the IEEE 1394 specification, also known as Firewire. National Instruments sells the DAQPad-6070E which can be purchased for the IEEE 1394 bus. It is rated at 1.25 MS/sec. This performance does not change when more Firewire devices are introduced into the system. You can check out more about Firewire at http://www.ni.com/tech2000
