It would probably be best here to compare French Guiana with the other "Guianas," namely Guyana and Suriname. Guyana was a British colony, while Suriname was a Dutch colony. Both of these countries became independent relatively recently; Guyana in 1966 and Suriname in 1975.

Beginning in the 1940's and 1950's France, the Netherlands and the UK began changing their formal relationships with their colonies. Instead of traditional colonies, the European countries wanted to give their colonies greater autonomy while simultaneously drawing the colonies into the political fold of the mother country. The idea of having a formal "colony" had fallen out of favor. In 1946 French Guiana became an overseas department of France, which meant it was politically formally part of France no different from any other French department. In the 1950's the UK began experimenting with Guyanese self-rule. It allowed for local elections, but after an alleged communist with ties to the USSR was elected the British government intervened to force him to step down. This was followed by a rift between the two main Guyanese political leaders that made local Guyanese politics rather treacherous. In 1954, Suriname became an official part of the Netherlands. The Netherlands also gave its former colonies a great deal of autonomy in its local politics during this time. In the came of the Suriname, it was actually the case that many Dutch politicians wanted to grant its colonies independence. After World War II many European colonies had lost their economic allure for the mother country, and were often looked at as a waste of money that provided little economic benefit. Dutch politicians in the 1970's made it a priority and official policy to give its former colonies independence. To my knowledge the British never adopted an outright pro-independence stance, but sentiments of colonies no longer being worth the effort certainly helped ease independence for many colonies, including Guyana.

One of the main differences between French Guiana and Guyana and Suriname in the 1960's and 1970's was population. In 1966, Guyana's population was 660,000, Suriname's in 1975 was 364,000; while French Guiana's population was 44,000 and 55,000 in 1967 and 1974 respectively. So French Guiana always had a much smaller population than the other Guianas, that was unlikely to be economically viable as an independent country. French Guiana was also chosen as the site for the European Space Agency in 1964 which helped to bring economic benefits as well as people with closer ties to mainland France.