Because there's only downside. If you are a CEO of a big company, it's far too easy to say something that can be misconstrued and have a negative financial effect. Even worse, if you start twittering about trivial things, some people might think the emperor might not have any clothes.

On June 6, Larry Ellison--CEO of Oracle, one of the largest and most advanced computer technology corporations in the world--tweeted for the very first time. In doing so, he joined a club that remains surprisingly elite. Among CEOs of the world’s Fortune 500 companies, a mere 20 have Twitter accounts. Ellison, by the way, hasn’t tweeted since.

Zuckerberg and Marissa Mayer tweet rarely.

Twitter is mostly for sports stars and celebrities.

You're right that there's no upside for CEOs.