Bradbury uses Mars and the Martians as a contrast to the way Earth people (primarily Americans) deal with the world around them. And, unsurprisingly, Americans don't come off too well. As Spender points out, that means they don't understand their own role in the environment. Moreover, The Martian Chronicles makes it clear that American-built technology won't last—either on Mars or on the Earth. Technology may be amazing, but it's no match for nature.
Bradbury sets the book on Mars as a way to reflect on the problems of America's relationship with the environment without making Americans feel too uncomfortable.
The Martian Chronicles shows people changing the world for the better, to make it more livable for humans.