To be honest I think it has less to do with popularity and a whole lot to do with the people who usually watch CNN not tuning in because the Mueller Report didn't give them what they desperately wanted. The same thing happened to CNN's ratings following Trump's election. The hard core left that watches them because they only want to hear news that conforms to their view of the world stops watching because they don't like the news that they're hearing. I think if you checked you'd see that CNN's ratings were probably up following the midterm elections because liberals wanted to gloat. Now they don't have much to gloat about...so they're tuning out.
Just my opinion though...
Reporting news, or pseudo-news is not at all required for ratings though. EVERYTHING in commercial broadcasting gets a ratings assessment whether it's fake wrestling, (real) sports events, selling jewelry, onstage paternity tests, what the weather is, rummaging through somebody's abandoned storage locker --- EVERYTHING. And they all get them for the same reason: to establish how much they can sell 30 seconds of commercial time for.
You don't need to "agree" or "disagree" with what the weather is, who da baby daddy is, what's in the storage locker or how those naked people stranded on an island are going to survive. But they all INVEST you with an emotional attachment, the purpose of which is to keep you hypnotized long enough to sell you a visit to Barfer King.
There's nothing about "news" in any of that because there doesn't
need to be. If the unwashed masses quit watching the idea of naked people forced to eat bugs on an island, it doesn't mean they have come to "disagree" with the idea of naked people forced to eat bugs on an island ---- it means it's no longer pulling their emotional strings. It means they're not as
invested as they used to be.
That's exactly why they all use teasers -- "coming up we'll tell you how your house could get swept away by a tsunami next Thursday, but first this word about deodorant". They're locking you in so they can make more money. That's all it is.
This is all cynical mass psychology. Always has been.