I don't think it makes sense to say that "the rest of South America became independent" as if it were a single moment. Venezuela became independent of Spain 150 years before Guyana became independent of Britain. And Suriname didn't become fully independent of the Netherlands until a decade after that! French Guiana looks less anomalous in that light.

So to be more specific, why didn't the French decolonize their portion of Guiana along with the Dutch and British?