Getting slammed with traffic on a cloud system is the stuff of nightmares. Suddenly you wake up with a huge bill, and the tools to manage and restrict the cost are miserable in cloud systems because that is something they don't find useful. It's extremely cheap to get a system with a high speed link that for most use cases you'll never come close to saturating and in the unlikely event you ever do, then something is wrong and your service going slow or failing is better than trying to keep up and getting a huge bill out of the blue. Maybe not for every situation, but for most.

Most of the truly skilled sysadmins I know recommend burst-to-the-cloud rather than pure cloud.