Climate change is discussed a great deal on HN these days; probably more than any other topic:
https://hn.algolia.com/?sort=byDate&dateRange=all&type=story...
https://hn.algolia.com/?sort=byDate&dateRange=all&type=story...
https://hn.algolia.com/?sort=byDate&dateRange=all&type=story...
https://hn.algolia.com/?sort=byDate&dateRange=all&type=story...
Any topic this widespread is going to produce many copycat and follow-up articles that add no significant new information, as well as many sensationalized articles that don't provide a basis for substantive discussion. Users tend to flag those. If they didn't, climate change wouldn't simply be the most-discussed topic—it would be practically the only topic on HN.
There are also cases of bad flagging, where a particularly substantive article didn't get the discussion it deserved, but these are not nearly as common as people jumping to the conclusion that a topic is being suppressed when they run across a flagged submission. Checking HN search is an easy way to vet that logic (though not as easy as not vetting it). Frequently it turns out that the story has already had significant attention. If, after checking that, you see a particularly substantive article getting flagged, you are welcome to let us know at hn@ycombinator.com. We sometimes turn off flagging in such cases.
Everybody feels that the topic they consider most important is under-discussed on HN. Actually, every important topic is under-discussed on HN, because frontpage space is the scarcest resource we have: https://hn.algolia.com/?sort=byDate&dateRange=all&type=comme.... There's no way around this on a site that exists for curiosity, because curiosity withers under repetition.