Pretty much the same reason that Apple charges so much for their products - they cornered a specific market and their customers are willing to pay for the benefits of using their brand name, whether they are merely perceived or actual notable benefits.
I've used a few different graphics tablets before and most cheaper ones do have notably lower quality accuracy/pressure levels or just build design - ie they tend to break easily or quickly, even with just standard usage and no physical damage done. The Wacoms have the benefit of being actual artist toolsets - they match the job of being a professional piece of equipment and if you're creating something to a commercial or professional level, then they're worth the cost. An iPad 2 isn't nearly as accurate or well designed for art creation as a Wacom Intuos.
However, if you're looking for a cheaper alternative, I'd recommend Aiptek's graphic tablets:
Aiptek Media Tablet 14000u
and
Aiptek Media Tablet Ultimate 2
I've had 2 of the 14000u's myself, due to the first one simply ceasing to respond, but they are very good tablets for more standard levels of usage (I did some graphic design and some art projects for myself and nothing more). But having tried out the Intuous, I can tell you very quickly that levels of detail and pressure, the accuracy, responsiveness, etc, etc are far higher, if you're willing to pay that much. Which I was not, unfortunately.