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)