People talk most about CO2 because it has the most total effect as a driver of climate change. CH4 for example has a greater effect per kilogram, but we emit a lot less of it and it doesn't last nearly as long.

H2O technically affects the temperature even more than CO2 but it's not a driver, because the total H2O in the atmosphere depends on overall temperature. Emitting more H2O, from hydrogen cars or something, would just mean you get more rain somewhere.