For the dragon, the immortality actually matters, because it actually kind of hurt. But for immortal, it doesn't matter because it's damage is almost negligible and it has no ability. The immortality is pretty much wasted. And now, there are enough things that could kill immortal creatures already. Lastly, you almost never see them anywhere except AI3 and AI5, one is deck designed to be beaten the other is randomly generated decks. When nobody uses a certain card, it becomes evident that it's underpowered.