And the answer is simple. Because it's not charging.
It's because the voltage of most older USB computer ports isn't powerful enough, although the iPad will charge while awake on some Macs.
The same happens with the iPad 2 as well as the original iPad.
So...if you can't use your mains charger and HAVE to use USB, just hit your power button to put the iPad into sleep mode and leave it...for...a...long...time.
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