Asbestos is carcinogenic because it's extremely fibrous. By enclosing asbestos you mitigate its deleterious effects on human lungs. However, you do not remove its insulation properties, which may not be shared by whatever is physically enclosing it.

Simply put: all you need to prevent fibrous material from entering the lungs is a physical barrier. But a physical barrier is insufficient to provide insulation. Sandwiched asbestos provides insulation that the enclosure material cannot provide on its own.