What astronomical constraint forces SpaceX to adopt a "massively compressed waterfall" approach for deep space elements like Mars missions?

The Earth-Mars synodic period (26 months)

The Earth-Mars synodic period dictates the optimal launch window for interplanetary missions, occurring approximately every 26 months. This constraint imposes a severe penalty on the development cycle compared to Low Earth Orbit (LEO) operations where iteration can happen in weeks. If a critical system fails during a Mars launch attempt within one window, such as testing the Mars-ascent engine, the entire program must wait until the next optimal alignment, typically two years later, to attempt remediation or a follow-up test. Because failures result in such long delays, SpaceX must compress the design, analysis, and testing phases for deep-space systems significantly before launch, leading to the described 'massively compressed waterfall' development style, demanding near-perfect readiness on the first try for many unproven systems.