Only states can have electoral colleges, and have voting representatives in either house of Congress. Territories have to apply to the US for statehood. Each state has its own state constitution (which used to involve things like slavery,) which it then submits to the Federal government for ratification. The Federal government has to wait for this application before it can create a state. For example, if you look at a map from before the Civil War (
link) you can see that there is a large swath of land in the middle of the country that wasn't states, though was theoretically American. Ignoring the fact that there were a lot of pissed off Native Americans in those regions, territories have their own set of rules. The rules are exactly the same in Puerto Rico as they were elsewhere.
Puerto Rico, to the best of my knowledge, has never applied for statehood.