There are a number of reasons why (some of which I'll cover later), but at the core is this: they have plenty of individually powerful cards, along with powerful tribal bonuses
Graded Population Report: Summon: Bahamut [Borderless Foil] #356 Visualize the distrbution of grades in the chart
Then there are a couple pieces of interaction (and anti-interaction?) between Swan Song, Heroic Intervention and Asceticism
The set included 62 total cards, and since the box includes 36 booster packs, each with 11 cards for a total of 396 cards, it's likely that it includes more than one complete set
It would appear that Draft Boosters and Set Boosters can have both foils and nonfoils of Showcase or Borderless treatments, but no Extended Art at all
Path to Exile will fill a strong role in a format that incentivizes making land drops every turn, and itll also push ramp decks back up in the metagame (something the format desperately needs)