What stopped Puerto Rico from being 51 first state?
The people of Puerto Rico have narrowly rejected statehood in plebiscite elections on three separate occasions since 1967. There may be a fourth Plebiscite in 2012 if the current governor gets certain legislation passed.
In order for a territory to become a state of the United States 5 things have to happen.
1) The territory has to have a constitution (Puerto Rico Has one of these)
2) The voting public of the territory must vote to ask for statehood. 50%+1 of the electorate must vote to request statehood.
3) The territory has to petition Congress (US House and Senate) to be accepted into the Union.
4) Congress (both houses) must pass a resolution accepting the Territory as a state.
5) The President of the United State must "certify" Statehood (sign the Congressional Resolution into Law)
A star representing Puerto Rico would be added to the US flag only if Puerto Rico were to become a State in the Union. At present it is a self-governing territory of the United States, not a State. The option to seek statehood has failed to win a majority in several plebiscites held in Puerto Rico, so, for the time being, there will be no star representing Puerto Rico.