It wasn't the "most important part". You clearly only saw these movies as comic book popcorn movies and nothing more.
The story is only one part of a movie, a movie doesnt even need to have an intricate story to be a great movie. You can have a movie about a simple story, even one that makes little sense, and it can turn out to be a good movie based on the acting and cinematography alone.
The acting, interactions, and goals and relationship of Tchalla and Kilmonger were not what was happening in Aquaman.
The details are what make or break a movie.
To say that Aquaman and Black Panther were the same movie is absurd. They weren't polar opposites, but it wasn't the same.
You can not like both movies for whatever reasons you want, but if you saw them as the same it's because you weren't paying attention and/or are wayyy oversimplifying