Like so many things in this life, blame Byron!

His poem "the Corsair" was written close enough to the peak of piracy that everyone would enjoy it and imagine the characters, but far enough removed from that time that people were able to accept his romanticism, so we're talking about the early 1800s.

This initial transition only happens in the rich, aristocratic and educated of course, the average person has little knowledge regarding pirates beyond their violent and criminal effects on coastal communities.

this new image of the noble outlaw was not a new archetype, however it was new in being applied to the specific vocation of the pirate. Other rebellious types had been held in high regard at different times in history, all the way back to Set in Egypt or Loki in the north.

From here the pirate moved in the popular understanding slowly but continuously, from a violent ruthless cutthroat to the rebellious free-spirit we see in modern media.

Not an historian but i reckon it might have to do with the British empire sanctioning many of them to plunder Spanish ships.