Because string theory does not make testable predictions. Many require a theory to be testable before it is considered science.
Good point.
Perhaps string theory is a hypothesis trying to work its way up the ladder. It is following the scientific method, and may or may not pan out. But that does not make it something other than science.
Some things just take a little time. We'll have a better idea of its validity in a decade or two. What's the hurry?