Because people are under the impression that the CO2 humans are putting into the air will cause a kind of positive feedback loop that will result in more water vapor and a rapid and uncontrolable increase in surface temps on the Earth. Climate models based on this assumption have not been mirrored in observation. In fact it seems that there are other factors within the atmosphere that create a negative feedback loop that balances everything out quite harmlessly. I suppose it's just a matter of when proponents of human-caused global warming come to terms with reality.

Because water vapour is totally harmless to the environment....

Water vapor is water in its gaseous state-instead of liquid or solid (ice). Water vapor is totally invisible. If you see a cloud, fog, or mist, these are all liquid water, not water vapor. Water vapor is extremely important to the weather and climate. Without out it, there would be no clouds or rain or snow, since all of these have their source in water vapor. All of the water vapor that evaporates from the surface of the Earth eventually returns as precipitation - rain or snow. Water vapor is also the Earth's most important greenhouse gas, giving us over 90% of the Earth's natural greenhouse effect, which helps keep the Earth warm enough to support life. When liquid water is evaporated to form water vapor, heat is absorbed. This helps to cool the surface of the Earth. This "latent heat of condensation" is released again when the water vapor condenses to form cloud water. This source of heat helps drive the updrafts in clouds and precipitation systems. Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for water vapor alone, and between 66% and 85% when factoring in clouds.

Because "human activity does not significantly affect water vapor concentrations except at local scales": "Water vapor accounts for the largest percentage of the greenhouse effect, between 36% and 66% for water vapor alone, and between 66% and 85% when factoring in clouds. Water vapor concentrations fluctuate regionally, but human activity does not significantly affect water vapor concentrations except at local scales, such as near irrigated fields. The Clausius-Clapeyron relation establishes that air can hold more water vapor per unit volume when it warms. This and other basic principles indicate that warming associated with increased concentrations of the other greenhouse gases also will increase the concentration of water vapor. In climate matters, when a warming trend results in effects that induce further warming, the process is referred to as a "positive feedback"; when the effects induce cooling, the process is referred to as a "negative feedback". Because water vapor is the primary greenhouse gas and because warm air can hold more water vapor than cooler air, the primary positive feedback involves water vapor. This positive feedback does not result in runaway global warming because it is offset by negative feedback, which stabilizes average global temperatures. One primary negative feedback is the effect of temperature on emission of infrared radiation: as the temperature of a body increases, the emitted radiation increases with the fourth power of its absolute temperature. Other important considerations involve water vapor being the only greenhouse gas whose concentration is highly variable in space and time in the atmosphere and the only one that also exists in both liquid and solid phases, frequently changing to and from each of the three phases or existing in mixes. Such considerations include clouds themselves, air and water vapor density interactions when they are the same or different temperatures, the absorption and release of kinetic energy as water evaporates and condenses to and from vapor, and behaviors related to vapor partial pressure. For example, the release of latent heat by rain in the ITCZ drives atmospheric circulation, clouds vary atmospheric albedo levels, and the oceans provide evaporative cooling that modulates the greenhouse effect down from estimated 67 °C surface temperature." Source and further information: http://en.wikipedia.org/wiki/Greenhouse_gas

water vapor doesnt effect any thing thing because its natrual.But carbon has all those hazardous chemicals.

Because industry produces CO2 and people like Al Gore hate industry.