Because as a general rule, if you're going to engage in commentary of a subject matter, you should know what you're talking about. Many people, including many scientists (certainly among the vocal ones), fail to mind the bounds of their narrow domain of expertise and tread unwittingly into philosophical territory, all the while believing their views are scientific in nature. This happens all the time with materialism, which is not a metaphysical position entailed by science, but nonetheless is often presented as if it were and sold to the public as a package deal. Sagan was certainly guilty of that, that and his whiggish view of history.