Magic was created by Richard Garfield, but for most of its 24-year history, the game’s design has been overseen by Mark Rosewater. Mark is excellent at explaining his design theories, and he often cites a trifecta of ideas that have made the game so good.
The first is the color pie. Each of the five colors - white, black, green, red, and blue - has unique strengths. Some capabilities are shared by two colors, many are just in one. The consistency of these traditions creates an environment where you know what to expect and how to plan - even when facing the unknown.
The second is the mana system. Each deck must include land, and can play one land per turn. When you amass 4 or 5 lands, you can use your more powerful spells. But you also have to plan for the parts of the game where you only have 2 or 3 lands. Land can also be used to activate cards already played, play multiple spells on the same turn, or pay alternate costs for certain spells. It’s a tightrope walk, since you don’t want to get stuck with too many lands and too few spells - but you definitely don’t want the opposite. Often, including spells that increase your access to mana, or simply draw more cards, is part of the balance.
The third innovation that set Magic up for success was the fact that it’s a trading card game. Pieces for a deck can be interchangeable. Often, your deck will benefit from having a few different cards that do similar things, but you could also swap some creatures for some control spells, for instance, or some artifacts or enchantments, and still have a coherent strategy. The relevance of one can make up for the loss of the other, and give your deck resiliency against varied threats. But when your strategy is too diluted, your limited access to cards can slow you down. If you devote a whole deck to working as an engine, you can draw tons of cards. But when you start refilling that deck with cards you care about drawing, the engine eases up or gets clogged. The importance of tuning is part of what makes Magic so fun: because complicated decks can work and the answers aren’t at all straightforward.