TV viewership in the US has been in decline for years. Now, mainly the older generation watches it. Hence, channels which target older audiences (e.g., CBS, Fox News) generally do better. Also, the Disney-Fox merger only added a few cable channels (e.g., FX, Nat Geo) to Disney's portfolio. Notably missing was the Fox broadcast network and any Fox-branded sports networks. Therefore, the merger wasn't expected to increase their TV viewership by much. It was, however, intended to strengthen their capabilities in terms of development and production. For instance, ABC Studios and 20th Century Fox Television are now both subsidiaries of Disney Television Studios.
In terms of movies, the Disney-Fox merger led to them accounting for 38% of the US box office in 2019. That's definitely not a failure by any means. In comparison, their closest competitor Warner Bros accounted for only 13.8%. As a side note, Disney removed the "Fox" branding from their acquired film studios, so 20th Century Fox was renamed to 20th Century Studios. Then, will Fox Star Studios eventually follow suit?