A source like Vox covers urban/coastal issues from the 'inside', treating first-hand sources as credible and writing like the reader is included in the topic. There's also a significant assumption that readers inherently care.
It writes rural white plight stories as 'explainers', approaching them the way you would approach some international crisis your readers are unlikely to know about. One big human-interest frame example to make your readers care, lots of background and statistics, first-hand interviews that add flavor, but are aggressively contextualized by talking to professors and experts who aren't part of the community. And, of course, a necessity to say "which maybe caused Trump" or otherwise justify why such stories are relevant to their readers.
Basically, I think NPR/Vox/etc have started covering rural plight stuff far more, but they're still explaining those people to their readers.
If you summarize every story, you end up with a lot more "affirming a liberal view of the world" ones than "affirming a conservative view."
That said, I still believe the content of the stories presented hews far closer to objective fact and truth than what I've seen from conservative media. So I wouldn't say it's nearly that biased.
But of course, 80% is actually pretty close to the number of Americans that are not white heterosexual men. We may just have gotten used to an overrepresentation in movies, politics, and our social life.
I think the problem is you're lumping all minorities into one, and treating rural whites as a separate category.
Pick any particular minority group, and you could make the same statement:
"80% of the stories of the day are related to non-X group. Group X is rarely mentioned"
It's common to watch for bias on a per-story level, looking for either bad facts or heavy spin. That's the level where liberals insist conservative media is biased and other media isn't. NPR is generally great on that level - if they run a piece, it'll be factually accurate and told from some basically reasonable viewpoint.
But it's also worth talking about multi-story bias - what will you come away believing if you hear 100 (factually accurate) stories from this source? That's the usual level where conservatives complain about liberal media bias. (They're not wrong, though conservative sources do plenty of the same.) NPR looks way less good on that level; the scheduled programs (e.g. Fresh Air) lean very liberal/cosmopolitan, while the news reports are prone to a focus on political, coastal, and international issues without so much awareness of the 'interior'.
It's still a vastly better source than most news for learning about any single event, I generally prefer it to NYT coverage even, but it's not exactly comprehensive.