Everyone here has said there peace on what is the cause of the TV+ being unpopular and I just wanna add my thought here: Only original programs.
That shouldn't seem bad, really, but the issue is, in this oversaturated market, what really reels in consumers is both original content and being able to see already made content. Apple TV+ only has Apple Originals and no other shows from previous years and other studios. And this is bad for them because having shows and movies that have already been made, aired and watched by many is a big factor in drawing people in. Most people, when seeing a platform with only new and unfamiliar shows, will be intimidated by this new thing they don't know about and, since the 4.99 a month only pays for these originals, would rather go to platforms with both new stuff and things they wanna rewatch.
But that's just my theory.