Why do you think the Aztecs allowed some conquered peoples to govern themselves with relatively little interference?

The Aztecs allowed some conquered peoples to govern themselves with relatively little interference to preserve resources while expanding their control. This process is called indirect rule: one group conquers another, but instead of spending a great deal of resources quelling resistance and establishing a government there, the conquerors allow the local governments to continue ruling; however, the conquerors (Aztecs included) often demand a tribute from these local governments. So while the local governments still rule, they are no longer sovereign and instead must answer to the conquerors if need be.