I am not sure where you got this information from. Chlorine Dioxide tablets originally had a shelf life of 3 years. That was increased to 4 years about a year and a half ago.

It should be noted that iodine treatments are not generally effective against Cryptosporidium, whereas Chlorine Dioxide is effective. That may or may not be an issue for the user depending on their concerns. Iodine has been used pretty effectively for a long time.

There's also the taste issue. Most folks find that Chlorine Dioxide has no taste whereas iodine tastes like, well, iodine. That can be solved with addition of ascorbic acid (vitamin C) after the treatment regime is done.

Every purification method has its pros and cons.