The residents of Puerto Rico cannot vote for a new president because even if they are a part of the US territory, this country is not considered to be a state of the United States. This sets apart Puerto Rico from Hawaii, which is also a bit far from the United States but those who live in Hawaii can definitely vote for the president that they want to have.
There are more than 90% of people from Puerto Rico who are hoping to achieve statehood in a few years or so because they believe that this will benefit the residents more. They may also be more advantages that will become available to the Puerto Rican people when this happens.