Movie trends are somewhat cyclical.
I think people are a bit tired of blockbuster/popcorn films that take themselves too seriously (the Matrix sequels, the new Star Wars, Lord of the Rings, Superman Returns, King Kong, etc.) and are craving something a bit more "fun".
Also "Pirates" is hardly the first huge film franchise that's been compared to a rollercoaster ride. Indiana Jones did huge box office in the 80s, I think Pirates has a similar type of appeal (though the Indiana Jones films are better).
The other reasons are casting. Johnny Depp has so much appeal across the board as this type of character. Usually guys have to drag their girlfriends to see these big "blockbuster" movies, as they tend to skew towards the male demographic.
But "Pirates" is something a girl and her girlfriends would go to see on their own anyway, without having to be dragged there by their boyfriends.
That's a big difference. Pirates appeals to the guys who want the usual summer blockbuster, but to the ladies as well. It also can play to the family audience. There really isn't a demographic it doesn't hit.