The first is that domestic interest began waning before global interest.
Starting with the fourth movie (and again please note sequels tend to do better than the first movie in a franchise until interest starts to wane), domestic interest began greatly wearing off.
Looking at the numbers above, there are a few things we need to address.
There is an outlier there in that if you count the global intake for the fourth movie, Dead Men Tell No Tales, worldwide it was still dominating and making over a billion dollars, but the studio takes in a much smaller percentage of box office totals in countries outside of the U.S., so the domestic downtick after #3 is really the first worrying sign.