Official explanation below: While many other cards reduce cost in a way similar to this card, this is the only card that can reduce cost without consuming any cards in the players hand or imposing any conditions, making it powerful in every stage of the game
Ready to defend the kingdom

Each Collector Booster will have the following contents: Collector Booster Info 4 Foil commons, common dual lands, or basic lands 2 Foil uncommons 1 Foil showcase basic land 1 Extended-art rare or mythic rare (45 rares, 8 mythic rares) 2 Foil rares or mythic rares, or foil extended-art rares or mythic rares 2 Showcase commons or uncommons, or a borderless card being previewed by Martin Juza on June 8 (5 commons, 6 uncommons) 1 Showcase or borderless rare or mythic rare (8 rares, 17 mythic rares including 5 versions of mythic rare Teferi) 1 Foil showcase common or uncommon, or a borderless card being previewed by Martin Juza on June 8 (5 commons, 6 uncommons) 1 Foil showcase or borderless card, any rarity (5 commons, 6 uncommons, 8 rares, 17 mythic rares including 5 versions of mythic rare Teferi) 1 Foil token In a pack of a Core Set 2021 Collector Booster, you are always getting a minimum of four rares or mythic rares, and about 45% of the time, you will get five

It boils down to three key factors: Limited Supply : The rarity and cap on card production make them scarce commodities over time
As such, the set is full of dragons with familiar archetypes and mostly straightforward mechanics
This specific card coveted among other promo cards is that Ishihara signed it, and theres only one gem mint in stock, boosting its value even further