Why is CO2 blamed for global warming when it has low heat capacity and low concentration in the atmosphere while water vapor, with much higher concentration and much higher heat capacity is the real cause?

https://www.quora.com/Why-is-CO2-blamed-for-global-warming-when-it-has-low-heat-capacity-and-low-concentration-in-the-atmosphere-while-water-vapor-with-much-higher-concentration-and-much-higher-heat-capacity-is-the-real-cause