First, USB ports are in general slower than Firewire ports.
Second, USB devices are designed in such a way that multiple USB devices can be hooked up to the same USB port on a laptop. This allows laptops to talk to multiple digital devices at the same time using the same port if they wish to.
USB, in fact, is the preferred port used by most digital cameras, cell phones, printers and external storage devices. Firewire is what some higher end digital video recording devices prefer since it requires the transfer of huge volumes of data.
