Because the speakers want us to. One of the most reliable ways to see a language rescued from the brink of extinction is for the speakers of that language to gain financial clout in their communities. Once they are comfortably positioned socioeconomically, they are free to launch their own efforts to keep children speaking their language and demonstrating pride in their heritage. That is, every people wants to keep their language because it is a huge part of their culture, but not every people can afford to.