What is the primary reason the $ ext{LH}_2$ fuel paired with $ ext{LOX}$ oxidizer yields exceptionally high Specific Impulse?

The exhaust products are very light, primarily water vapor.

The superior Specific Impulse ($ ext{I}_{ ext{sp}}$) achieved by the liquid hydrogen ($ ext{LH}_2$) and liquid oxygen ($ ext{LOX}$) combination stems directly from the physics of the reaction products. Hydrogen is the lightest element, and when combined with oxygen and combusted, it produces water vapor, which results in an extremely low molecular weight exhaust stream. According to performance principles, lighter exhaust molecules moving at high velocity lead to significantly higher $ ext{I}_{ ext{sp}}$ values, meaning more efficiency per unit of mass consumed. While this combination provides peak efficiency, the trade-off is the logistical difficulty associated with maintaining liquid hydrogen at temperatures below $-253^ ext{C}$, causing rapid boil-off.