Why? There are quite a few complaints regarding this. The iPad is a pretty sleek and unique device. There's not a lot of room inside it for high powered hardware. The additional features that are in 6 that are not in 5 need hardware that simply does not exit in the original pad.
It's due to the RAM and processor limitations.
iPad: RAM - 256MB, Processor - 1 GHz Apple A4
iPad 2: RAM - 512MB, Processor - 1GHz dual-core Apple A5
iPad 3: RAM 1024MB, Processor - 1GHz dual-core Apple A5X
The answe? Simply just enjoy your iPad with whatever OS it's got on it. iOS 5 is great,
and even those on iOS 4 are functionally usable as well. Just because
Apple doesn't provide updates doesn't mean you have to throw your device
away. Some of us have still got an original iPhone running OS 3, and it is still a
beautiful device that does its job.