Here's why:

* Effectiveness: Chlorine is highly effective at killing bacteria, viruses, and other harmful microorganisms.

* Affordability: Chlorine is relatively inexpensive to produce and use.

* Stability: Chlorine remains effective in water for a longer period of time, ensuring continued disinfection.

Other gases used for water disinfection, though less common, include:

* Ozone: Ozone is a powerful oxidizer that kills microorganisms quickly. It's often used in conjunction with other disinfection methods.

* Chlorine dioxide: Chlorine dioxide is another strong oxidizer that's effective against a wide range of pathogens.

While chlorine is the most common, the best gas for water disinfection depends on factors like the source of the water, the level of contamination, and the desired level of disinfection.