Because the average computer does not provide enough juice via USB to charge the iPad battery. You may have noticed in the box with your iPad was a larger (10W) charger. That's what you should use to charge your iPad. Kind of handy too, as it doesn't require you be anywhere near a computer to charge your iPad.
Your computer isn't a Mac.
More specifically, the USB standard specifies that the output for a USB port is 5V at 500 mA for USB 2 (and raised to 5V at 900 mA max for USB 3). According to the USB standard, a low-power device may only draw 110 mA (also called a "unit load") which is the default unless the device negotiates with the USB controller to receive more power.
On Macintosh computers, the USB ports permit devices to negotiate up to a 10-unit (1100 mA) load on the port. The iPad 2 comes with a 10W charger (5V x 2000 mA) and "charges" when plugged into a USB source that provides at least 5W of power (5V x 1000 mA).
Conventional home computers generally do not have USB hardware capable of supplying more than the standard 2.5W (5V x 500 mA) of USB 2.
You should use the charger provided with your iPad. However, if you plug in your iPad to your USB port, it will charge, just very very slowly (compared to the proper charger).