Puerto Rican here, here are answers to your questions. I tried to be as impartial and nuanced, but it's also good if you do your own independent research.
- Many people within the Pro-Independence movement will claim that the Gag Law of 1948, which was pretty much the Spanish version of the Smith Law was the reason why the Independence movement never caught on. This view is very simplistic and incomplete. Yes, there were government crack downs and Nationalist leaders where persecuted but that alone was not enough to take the oxygen out of the movement. The electoral history of the movement shows this. In 1948, the first real general election in PR where Puerto Rican could vote for their own governor, had the PIP coming in 3rd place with 10% of the vote. In 1952, the same year the Commonwealth status was put into effect, the PIP did even better being the runner up for the governorship by winning 19% of the vote. In 1956, the PIP party once again returned to being 3rd with a respectable 12% of the vote. It's not until 1960 that the PIP party takes a nose-dive coming in at 4th with 3% of the vote, behind a Christian Party that had made its debut that electoral cycle. In other words, despite the Gag Law being in effect from 1948 until it was struck down by the courts in 1957, the PIP party was able to have a strong showing in elections. It's not until 1960 when they become a small presence in the political scene. There are several reason as to why this happened.
- The PPD Party was able to clearly define a political relationship with the United States by becoming a commonwealth. This political status was masterfully crafted by the ruling PPD party because it calmed fears of losing Puerto Rican identity and autonomy while simultaneously keeping a close relationship with the USA. It was the best of both worlds and people at the time liked it. For the tome being, the status debate was settled; PR would be neither an independent nation nor a state but a mix of the two.
- The ruling PPD party also rolled out massive economic reforms that, for the first time, would strive to Make PR an industrial hub. This massive push towards investment was called "Operation Bootstrap" and it fundamentally overhauled Puerto Rican economy and society like never before. For the first time, roads were being paved, jobs outside the agricultural sector were much easier to get, women were able to become more independent, diseases where being tackled through mass vaccinations, schools were being built, universities were being overhauled. Utilities were being built in cities and in the countryside. People, with their own eyes, saw progress for the first time.
- LBJ's the Great Society initiative gave Puerto Ricans access to federal programs that further helped PRs development. It helped reduce poverty even further. Such progress also signified that PR was finally seeing the fruit of the ELA status and showed people the importance of a relationship with the US.
- When after a Schism within the PPD happened in 1968, the first pro statehood governor was elected and set out to further build on what LBJ had done nationwide by implementing a state safety net which solidified his Party (the PNP) as the other major party alongside the PPD.
TL;DR: A defined relationship with the US, better economic outlook, benefits from the federal government by way of the Commonwealth status, and the creation of a local social safety net ended all of the main arguments that the PIP party had used to win elections in the past. Things were moving forward, why rock the boat?
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As mentioned above, the independence movement up until 1960 had significant support but never majoritarian support. Statehood was always generally more popular than independence but it wasn't until 1968 that the stars aligned for the pro-statehood party to win the Governors mansion. And it wasn't until 2012 that statehood began to take in serious support in plebiscites and it wasn't until this past plebiscite that Statehood was able to win an outright majority of the electorate in a "Yes" or "No" vote on statehood.
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Other than symbolic gestures from countries that had grievances with the US, no. There was never any concrete support from foreign countries in favor of Independence. PR's economic and social progress was, at the time, the US' pitch to Latin American countries to join its side of the cold war.
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Ironically, members of the PR diaspora tend to be more vocal about PR independence. However, don't let the vocal diaspora fool you. Though polling is scarce on this issue, an NBC poll conducted ahead of the 2020 elections showed that Puerto Ricans living in the diaspora supported statehood, albeit by a lower margin, than those living on the island. Here's the poll if you want to give it a look.
https://www.americanprogressaction.org/issues/democracy/reports/2020/09/24/178839/puerto-ricans-really-think-ahead-2020-elections/