Why both? First, illustrator doesn’t support multiple pages and master pages, but inDesign does. Also, inDesign doesn’t offer drawings and icon libraries, but illustrator does.
Getting Started with Wireframes
Nicole Saidy

It used to not be this way, since Adobe CC Illustrator offers multiple pages and layers that can result in masters. And InDesign, which is fantastic for wireframes does offer the drawing tools necessary for wireframes, since wireframes are not extremely detailed. Furthermore, InDesign does offer libraries.

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