Only US States are recognized in Congress. Puerto Rico, among other places is a territory which gives it a delegate to congress, but no direct participation. It's also not considered sovereign, which US States nominally are.
This has advantages and disadvantages... there's no tax, but also no Senators or congresspeople who give a hoot. Sometimes congressional delegations from places with big Puerto Rican populations will push for policy to secure votes on the mainland.
Also remember that it's a colony, and it's standing is colored by the baggage of colonialism, and there is a split between people who favor that status quo, statehood and independence.
Statehood will never happen unless there is a Democratic supermajority in the Senate.
