Here's why:
* Dome Fossils are specific to Pokemon SoulSilver. In SoulSilver, you can find the Dome Fossil in the Dragonspiral Tower on the third floor, behind the entrance.
* HeartGold features the Helix Fossil instead. This fossil can be found in the same location as the Dome Fossil in SoulSilver, Dragonspiral Tower on the third floor, behind the entrance.
If you're looking for a Dome Fossil, you'll need to play Pokemon SoulSilver.