An opinion piece in The Hill says:
That's what happens when all-things-Trump are myopically put at the forefront of every offering: You take J.R. Ewing off "Dallas," and there's no more "Dallas." You take Trump off the stage, there's no more cable news that has existed since June 2015, no more late-night "comedy" on CBS, NBC and ABC. It's the main (albeit not only) reason why CNN has lost nearly 70 percent of its audience since the beginning of the year, while MSNBC is down more than 40 percent.
Can this claim of causality be substantiated somehow? First, I'm guessing that the events in January had a lot of people "glued" to their TV, so perhaps a year-on-year comparison would be more appropriate. Second, does this fit some kind of general pattern of left-leaning TV/media being less watched during Democratic presidencies?
I'm not personally skeptical of the precise figures given below from Carolina Coast Online--thanks Joe W for editing these in--, but they were linked in the Hill piece to justify the 70%. I suppose it's the fact that Fox News lost substantially less audience that's part of the claim/evidence.
CNN’s drop in viewers is larger than its competitors, but all major cable news shows have experienced a drop. MSNBC had the second-largest drop in viewership since January, losing 49% of its total viewership between January and May. In the 25-54 age demographic, the network lost 63% of its viewers. During primetime hours, MSNBC lost 42% of its total viewers and 58% of viewers between the ages of 25 and 54.
Fox News had the least substantial drop out of all networks, losing 12% of its total viewers and 15% of viewers between the ages of 25-54 between January and May.