From the Daily Beast:

Insiders also claim not to be surprised that the conclusion of the long-awaited Mueller report—or at least the Trump-appointed attorney general’s summary—was a whimper, not a bang for an outlet that has invested so much time and energy, in primetime and throughout its dayparts, in the notion that Trump is unworthy of the Oval Office and might at some point be forced to give it up.

And it’s also possible that the Mueller disappointment drove loyal viewers away in much the same way that people avoid looking at their 401(k)s when the stock market is down. Maddow, who has consistently vied for the first or second top-rated cable news program, was sixth on Monday evening, down almost 500,000 total viewers from the previous Monday, as was MSNBC’s second top-rated program in primetime, The Last Word With Lawrence O’Donnell.