I have always understood iodine and chlorine dioxide water disinfection to be quite effective, when used properly.

And "when used properly" may be the key issue: when making recommendations for a mass population, you basically aim for the greatest good with the least harm. Let's face it, some people don't sweat the details in normal times, and in crisis that won't change for the better.

Iodine used improperly can be ineffective. It can also be poisonous. Anything in tablet form presents a poisoning hazard to children. You get the picture.