It's not like the iPad 4 was a heavy device to begin with. The previous generation weighed 662 grams, the iPad Air weighs just 478 grams.
The reason why Apple is doing this is because as a culture, America and most of the western world is obsessed with the idea of “thin” and “light” to an almost unhealthy degree. They are producing precisely what the buying public wants, even if it compromises the overall durability of the design.
I'd quibble with Jason's assertion that Apple is obsessed with thinness and lightness to the detriment of other aspects of the device. I'm sure the company could have made a device as thin as the iPad Air in the past, but it didn't because that would have compromised on things which Apple holds much higher in the scale of importance than mere millimeters: battery life and thermal performance.
But to the extent that lightness is a factor Apple focuses on (thinness is just a method of removing weight and achieving the right balance), it's down to it being something which allows the device to disappear from view. Apple has consistently aimed to make the technology get out of the way, to let people do things without having to focus on the device. The bigger and heavier something is, the more the device distracts from what the person is trying to achieve.