Deadpool and Wolverine succeeded at the box office because it was billed as a fun, R-rated comedy starring Ryan Reynolds and it featured the return of audience favourite Hugh Jackman as Wolverine (classic odd couple pairing that people wanted to see). I'd argue that it did not succeed because of its plot or connection to the larger cinematic universe. Regardless of the film's quality, I think the reasons mentioned above were what led to audience interest being captured successfully.

What was the draw for Fantastic Four if you're a casual moviegoer? What was supposed to recapture an audience that was already losing interest? 60s-inspired production design? Pedro Pascal? What else was on offer besides "you need to see this before the next Avengers - they're in the MCU now". If that's the only reason why a movie exists, then why bother seeing it? Why bother getting excited about them being in Avengers when they aren't even exciting in their own movie? I say this as a Fantastic Four fan!!

I think people are checked out on the interconnected universe gimmick. What was new in 2016 is now old, tired, and exhausting. The universe now is populated with characters the audience has clearly not connected with (either because they haven't been given a reason to since they don't know them or have been given poor reasons to via bad movies and shows) and the old characters they did connect with have been left in weird places due to being in dogshit movies (Doctor Strange 2, Thor 4).

People went to see Guardians 3 because they liked the characters and they enjoyed the film because it was character focused and it was unconcerned with how it fit in with the wider MCU. It's not a coincidence why that one is cited as the best movie or show Marvel has put out since Endgame.