I studied the history of the Dom-Tom territories during my history undegraduate diploma, maybe I can give you some informations. I haven't looked at the subject in a while so maybe someone else will be able to give you decent sources, tho, for I can't find them. They were in French, qo I don't know if they'd be of any help to you.

You have to understand that compared to the other French Union territories that composed the french colonial empire, Guyana was a bit special.

First, unlike other territories, it had nearly no indigenous population to fuel an independence movement at the time compared to say the african colonies which were composed with a 90-98% local population and 5-8% colonists at most. Most of the natives were decimated in the last century and were secluded and isolated deep into the guyanese rainforest. In the sixties, they were roughly 2000 of them left. While considered integrated to the Union on paper, they were basically not part of the society. Most of the territory's population, compressed on the coast, was comprised of french citizens descended from slaves, indian displaced workers or from metropolitans french.

another important point : Most of the local elites were from metropolitan France, with nearly none of them being from afro-guyanese, indian or native origin, compared to the Indochinese or West African old colonies, for exemple, where local elites were usually well-educated and politicized. This changed over the course of the second half of the XXth century with the gradual politicization of Guyaneses from indian, african origin (and one of them became one of the most known Justice minister of the Vth French Republic, Christiane Taubira)

Second, it was a small territory, demography-wise, compared to the other parts of the former french colonial empire. At the time of the decolonization process, the former penal colony of Guyane had the population of a small french city (roughly 28k inhabitants), and had long been devoid of any real capability in reaching self-sustainability (by design, it was a penal colony), in any way, with little to no infrastructures, economy and food sustainability. It was entirely dependent on metropolitan France for a lot of things.

And the last and third point, the deal they got with the creation of the Union. They became a full-fledged département, with all the pros (representation in the French Parliament, State obligations concerning the well-being of local residents, economic help) and cons (limited autonomy) coming with it. And at the same time, de Gaulle, looking for a new test site for the french space program after France lost the original ones in Algeria, saw a lot of advantages with the guynase situation : nearly no population, close to the equator, a big ocean under the course the rockets were supposed to fly. Relocating the space program here came with the promise to boost the local economy and invest a lot in the local infrastructures.

So to say it roughly : it's a mix between the demographics (Nearly no indigenous population, no real "grassroot" political program due to the small population and local elites fiercely loyal to the mainland), logistics (a territory dependent on the mainland), and political and economical advantages conceded by the mainland more appealing than the prospective of independence.