Why doesnt the Ipad charge on the computer?

It does. Just let the screen turn off. The USB port doesn't provide sufficient power to charge AND display the screen. The wall charger that came with the iPad is much faster.

Welcome to Apple Support Communities.

Likely, your computer does not have the required high-power USB port.

For more details, see:

http://support.apple.com/kb/HT4060

and this included reference to 'high power' (defined as more than 500 milliamps output) USB

http://support.apple.com/kb/HT4049?viewlocale=en_US

Confirming what the other poster says, my older MacBook, lacking high-power USB, does charge the iPad 2 when connected despite the message, although at a very very slow rate. I've noticed that the iPad charge increases a by percent or two when connected for an hour or more.