You can only get a plurality in most cases with mainstream media, because like it or not it does tend to be progressive/liberal in most outlets (in the American sense, not European).

That doesn't explain why FOX is twice as watched as MSNBC.

NPR's drivetime radio shows have bigger audiences than Fox, for almost all of its shows (and sometimes, for all of its shows).

Cable TV is still a niche.

Probably because the Cathedral of mainstream liberalism has been mortally wounded over the past decade. That has rapidly accelerated as of late and you're seeing the last dying breaths with the rise of populism on the left and right. Maddow used to have much higher ratings. AOC for example is not part of mainstream liberalism. She is at the vanguard of mainstream popular leftism.