Information on lack of iOS 6.1.3 jailbreak

Hey everyone,

In my quest to increase my control over what my iPhone 4S can do, I've decided to jailbreak it, only to learn that there is no jailbreak for iOS 6.1.3. Some sources seem to say that a 6.1.3 jailbreak will never happen, but I'm unclear as to why. Does someone have more detailed information on this?