Why does chlorination still work?
The reason chlorination still works is that it is aspecific, just as @AliceD mentioned.

The way chlorination works is mainly by oxidizing biological molecules. Chlorine, along with other halogens, is a strong oxidizing agent. It breaks down the phospholipid bilayer membrane, proteins and enzymes. There is no way organisms can overcome this, unless it uses none of the chemical molecules that react with chlorine, which is virtually none.

On the other hand, antibiotics inhibit particular enzymes and proteins in a particular way and by making one or two amino acids different in the protein, the microorganisms can easily overcome this.

Reference - http://www.sswm.info/sites/default/files/reference_attachments/TUE%202011%20The%20Chlorine%20Dilemma.pdf