Likely due to the Rhystic Study
Of course, his card being enchanted in rarity helps that a lot, but he's also a valuable card that can halt your opponent's progress and win you the game
The Good Currently bugged on Magic Online (so it reduces mana costs by two of any combination of black, red or generic mana), which makes it way more powerful than it should be (for example, we can cast a Fanatic of Mogis for free with two copies of Ragemonger on the battlefield)
It contributes to the self-mill plan, and can bring back Arclight Phoenix singlehandedly for five mana
He also can move to any location for free whenever you play them, meaning with the location you put out for free this turn, hell effectively be allowed to head there straight away on the turn you played him for no cost
Amber and Amethyst likes to quest early and often. Emerald and Ruby is especially good at delaying your opponents progress early on. Sapphire and Steel is great at quickly increasing the amount of ink in your inkwell. Those descriptions from the quick-start rules for each deck proved true