Part of the issue is that there is no simple Why. To invent one is to either petition history or invent some perspective. It's not a bad idea to do these things, didactically, but they're unnecessary for the material. Or, said another way, there's something to be said for discovering your own "why" through familiarity with the many wonderous properties the complex numbers enjoy.

That said, that's a frustrating answer. An excellent book which does just what I said above and tells a lightly fictionalized "just so" story of the "history and development" of mathematics as an excuse to introduce everything in a motivated fashion is MacLane's Mathematics: Form and Function which I just recommend endlessly.

https://www.amazon.com/Mathematics-Form-Function-Saunders-Ma...