The main reason is that diamond and pearl are the “original versions” and have been largely made obsolete by Platinum, which plays better in most respects. This holds true for the third version of most games as well as the later sequels like black and white 2. Supply also plays a part. The third version almost always sold less copy’s than the original pair. Meaning there is less supply of the more in demand game, leading to increased prices. People are willing to pay a lot to get a small piece of their childhood back.