I am BGG's official honey trap
I don't think Piracy is really all that huge a theme.
Nor is sports.
Actually... Piracy is huge but just seems to fly under the radar with lower ranked games.
Since the start of 2015, according to the database, when it comes to ranked games, there have been more Pirate themed games (28) than zombie (17) and Cthulhu Mythos (15) themed games.
Piracy themed games are dominant against the other two mentioned theme types right up until 2010-2015 where Zombies pulls ahead (+20).