It's just the way the market is right now. Mobile devices, especially tablets, are very new, and the technology has been advancing at a tremendous rate. Buying a four year old tablet is like buying a 10 year old computer. You can get basic stuff done, but it's not going to do what the newer devices can do.
Apple has quadrupled the RAM in iPads since the first generation, and doubled the CPU speed every single year over the last four years; with the last year seeing them move from a 32bit processor to a 64 bit processor. That doesn't even account for the new, better hardware: screen resolution, LTE radios, faster wi-fi, etc.
It is slowing down. People have been hanging on to their iPad 2's for a couple years without too many problems. The iPad 3 and 4 are still good tablets to have. I expect the iPad Air will do me fine for at least two years. It will be the first time I haven't upgraded every year.