So much misinformation here, makes my eyes hurt.
http://www.taxjusticeblog.org/archive/2016/08/puerto_rico_and_section_936_a.php#.WRGm4fkrJhE
Prior to 1996, there was all sorts of industry in Puerto Rico: electronics, pharmaceutical, manufacturing, etc. Then the Small Business Job Protection Act happened. If you know anything about logistics, bringing industry to an island in the middle of the Caribbean is not cheap, so tax cuts were given to all the US and foreign companies to operate out of PR. Once those tax cuts were gone (thanks, Clinton), and there was no other regulation to take its place, all major companies left, leaving an enormous portion of the island unemployed. That's when people started leaving. That's when I left.