Why can't Puerto Ricans vote for president?

Puerto Rico is a Commonwealth of the U.S. Although Puerto Ricans are U.S. citizens by birthright, residents of Puerto Rico pay no federal income tax, nor can they vote in presidential General elections much like the residents of Washington, DC. Residents can vote in the primaries and send delegates to both the Democrat and Republican conventions. It would require an amendment to the US Constitution to give Puerto Rico votes in the electoral college or they could choose to become the 51st state. Puerto Ricans who reside in any other state can vote for president.