The problem is that obesity is a complex problem. Not everyone who is overweight is a lazy person who eats Doritos on the couch wile watching daytime tv and wondering why they’re so overweight.
Smoking was easy: with extremely rare exception, smokers get lung cancer. Period. Smoking is one of the very few personal behaviors that basically guarantees you will get cancer. It also has the side benefit of annoying the living daylights out of people who don’t smoke which creates social pressure against it. It was pretty much a slam dunk.