Would you be willing to sacrifice your batteries health in order for it to charge faster?
The more power you put into the batteries in order to decrease its charging time, the faster you will need to replace the battery/shell out the $299 for an replacement iPad.
To people who replace the iPad every year, that may not be an issue. But, to people who plan to keep their iPad for more than an year, they may not want to take that sacrifice to save 1-2 hours of charging time.....
Of course you could get a replacement for up to two years with AppleCare+ if the battery were to fail.