Soul's Fire is a Instant card in Magic: the Gathering MTG
The "Mox" name commands a certain respect in Magic: The Gathering, respect that the steep drawback of losing a card from your hand will do very little to tarnish
Magic: The Gathering is the original collectible card game, meaning it was the first game to combine the traditional collectibility aspects of baseball cards (i.e., cards with varying rarity, bought in randomized packs) with the imaginative play of games like Dungeons & Dragons (i.e., you get to fight goblins)
This means that Exploration tends to lose its effectiveness pretty quickly unless you have reliable ways to continue hitting land drops every turn
A five-mana 5/5 with absolutely no abilities used to be pretty darn good on its own
This allows you to repeatedly cast the same card over and over again, allowing for a deterministic win with Brain Freeze