I don't know if there was any single silver bullet bringing this one down, I'm sure it came down to a number of things coming together. My two cents is these movies have to look like a winner from the get-go, lackadaisical marketing or a movie that looks just serviceable isn't going to cut it.
How exactly was that done? I don't remember anything suggesting you had to watch the shows with the other characters (just knowing there were shows would seem to imply some familiarity).The mismarketing made people think they had to see Ms Marvel and WandaVision and Secret Invasion to understand the movie, and the smear campaign against Secret Invasion turned folks off that idea.