USB is easy, its hot-pluggable, does not need thumbscrews to fasten, and its a standard that can support many many things - from mice to scanners to printers to webcams to keyboards to gamepads. Its also relatively fast compared to serial/parallel, and is very expandable thru USB hubs, which come integrated into many things these days including keyboards and monitors.
Also USB has been around a while - Most pentium boards have USB ports, so its not a new thing that people need to buy connectors for.
They would have to hold up alot less standards if everything was USB.
Say you had a laptop with one serial and one parallel port. What could you do with it? Maybe a mouse and a scanner?
Now say you had a laptop with one USB port. You could run a printer, mouse, scanner, upload things from your digital camera, plug in a gamepad for gaming on the run, hook up a full sized keyboard or even hook up a cable modem to it if you dont have a NIC.
You could hook up a 4 port hub to that if you wished, and run four different USB devices.
Many different possibilities.