Why did Salesforce get rid of S-controls?

Salesforce got rid of s-controls in favor of two new technologies, Apex Code and VisualForce.  We prefer these technologies because they provide a well-defined API where we can maintain backward compatibility as we make regular changes to our UI, schema, and API.  The API is versioned so even if we make changes to it, older versions continue to work just as well.  S-controls may have had more flexibility as they were free to do what they wanted on the client-side, but they also frequently broke as we made simple changes to the platform.