What is the key enabling factor for SpaceX's dramatic reduction in marginal launch costs?

Successful implementation of reusable rocket technology like the Falcon 9 first stage.

The single most compelling argument for the necessity of SpaceX centers on its pioneering and successful implementation of reusable rocket technology, specifically involving the Falcon 9 first stage. Before this innovation, launch vehicles were almost entirely expendable, meaning costly multi-million dollar assets were destroyed after a single use, leading to an unsustainable cost structure for frequent missions. By treating the booster as an asset intended for refurbishment and rapid reuse rather than a disposable shell, SpaceX achieved a dramatic reduction in marginal launch costs, which subsequently propelled them into dominance over many legacy providers. This capability fundamentally shifted the economic calculus of accessing space.