Because moderate social pressure can be a good way to change habits? If you regularly see people drinking soda on a daily basis and hear nothing against it, it's a lot easier to fall into the habit. If you hear about people who stopped drinking soda and don't miss it, it might inspire you to do the same.

I'm not saying that no one should ever drink soda; just that more moderation should be encouraged, people should start treating it as a choice that you should make as a responsible adult and use in moderation, like caffeine or alcohol, rather than a default that everyone does starting as a kid and continues doing whenever they feel like.