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To expand on Magic being "hackable": The game is this way because every rule of the game can be changed by a card. There are no set in stone rules in the sense that they will always apply the same way no matter the gamestate - there are cards that say "You can't win the game" or "End the turn", "Skip the upkeep phase", etc. The game isn't just about playing by the rules effectively, it's also about changing the rules effectively and dealing with how your opponent is doing the same. I think a related reason the game exploded is the fact that design (deckbuilding) is an integral part. Having a hackable game is great, but if the hackableness is random it's not that exciting. It's when you can design an exploit (deck) and improve on it that it gets really exciting.

To me, the main reason Magic is fun is the variety of skills and approaches it encourages. You can play it as several different resource management games: there are at least 4 important resources to manage (time, mana, cards, life) and you can win by focus on getting one or more of these resources more or by preventing your opponent from having/using any resources, and all of those decisions have major impacts on your play and deckbuilding style. You can also play it as a game of assembling your crazy exploit of the rules fast enough to win (combo decks), or you might even be able to exploit the rules so hard you're essentially playing a different game from your opponent, so you have a completely different win condition to achieve (legacy Dredge).