I cannot discount the possibility of the fossile fuel lobby being somehow involved in this, but from a standpoint of pure sensationalism I can see why you would rather cover Chernobyl than the 2016 Great Smog of Delhi. The former involves a lot of topics with which the average viewer is barely familiar but still finds interesting, such as nuclear physics or engineering failures, so there's a lot of rabbit holes to be dug into. Meanwhile, the origins and effects of pollution are widely understood, there really isn't anything new to be said, and so a news story about polluted air killing 2 million Indians every year isn't so much interesting as it is just sad.