While charge density waves (CDWs) are a hallmark of the kagome metal $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$, their absence in the bilayer counterpart $\mathrm{Cs}{\mathrm{V}}_6{\mathrm{Sb}}_6$ remains a notable exception, with the underlying mechanism yet to be elucidated. Here, integrating ultrafast spectroscopy with first-principles calculations, we demonstrate that the lack of charge orderings in $\mathrm{Cs}{\mathrm{V}}_6{\mathrm{Sb}}_6$ originates from a synergy between an altered electronic structure and fundamentally weakened electron-lattice coupling. Experimentally, coherent phonon dynamics exhibit purely conventional anharmonic behavior without CDW-associated anomalies, while electron dynamics reveal a markedly reduced electron-phonon coupling strength (i.e., 0.13 in $\mathrm{Cs}{\mathrm{V}}_6{\mathrm{Sb}}_6$ versus 0.88 in $\mathrm{Cs}{\mathrm{V}}_3{\mathrm{Sb}}_5$). Consistent with these experimental observations, first-principles calculations indicate an absence of Fermi-surface nesting and a shift of Van Hove singularities away from the Fermi level in $\mathrm{Cs}{\mathrm{V}}_6{\mathrm{Sb}}_6$. Together, our results establish a unified physical picture in which suppressed electronic instabilities and weak lattice coupling preclude CDW formation in bilayer kagome metals.

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• Charge density waves
• Electron-phonon coupling
• Phonons
• Kagome metal
• First-principles calculations
• Ultrafast femtosecond pump probe