Here's why:
* Pokemon Crystal is a Game Boy Color game. The DS Game Boy slot is designed for Game Boy Advance (GBA) games, which are a more advanced generation of games than Game Boy Color.
* Game Boy Color and GBA cartridges are different sizes. The DS Game Boy slot is too small to fit a Game Boy Color cartridge.
You can play Pokemon Crystal on a DS if you have a DS Lite or original DS with a Game Boy Advance slot. You can insert a Game Boy Color cartridge directly into this slot.
However, you cannot play Pokemon Crystal on a Nintendo 3DS or Nintendo 2DS as these models do not have a Game Boy Advance slot.