Answer:
Puerto Rico was added to the US after the Spanish American war and the economic discrepancies between their original agrarian culture and the industrial US caused many of them to migrate to northern cities in the US in search for jobs like street vending.
The culture is a little different and its similar to the situation with people from north west China moving to major cities.
Puerto Rico, by federal law, belongs to but is not part of the United States. We were made citizens because the US wanted to protect its strategic military location. As in all colonies, we are here to provide benefits to the empire. Before, we were good for military reasons. But with new technologies in warfare, our role is now economic (we are a very important market for US goods). But, however important we are to the US economy, we will never be given representatives with voting powers or senators or the right to vote for the presidency.
To me, that is second class citizenship.