Why couldn't they become a state, but for the fact that they don't want to?
The Fed has its headquarters there and if Puerto Rico became a state, they would be required to pay taxes.
"The Fed," as in the Federal Reserve System, what do you mean? Yes, I understand that statehood would require federal income taxes, but how does the U.S. benefit from the current commonwealth, territory status?
If they don't want to be a state, why shouldn't we cut them loose? Look at all the money we would save on payments for social services, welfare, illegal aliens, Social Security, Medicare, etc.
Since FDR declared the Federal Government bankrupt in 1933, it has been in receivership to the Federal Reserve System. The Fed is a privately owned corporation operated for profit by 4 large banks. The people of Puerto Rico can vote all they want but the Congress is required by law to follow the dictates of the Fed. The cost of maintaining the offshore tax haven for the Fed is none of their concern and they will maintain the status quo for the foreseeable future.
Google FDR and bankruptcy for more information.