Because Brazil always had more than enough tropical jungle and risking a war against a major European power to add a few thousand more square kilometres of tropical jungle didn't seem like a good idea. read more
Brazil did annex French Guyana during the Napoleonic wars. Portuguese conquest of French Guiana. But the Vienna Congress gave it back to France. Any invasion of a English colony (Guyana) or a Dutch colony (Suriname) would be frowned upon and give Brazil big problems (probably an war against industrialized nations). read more
Guiana is a Department of France and no longer a colony. Its political status is similar to the state of Hawaii. The French sent 12,000 settlers to Guiana in 1763, but within a year only 2000 were still alive. read more
At the 2014 census, 57.3% of the inhabitants of French Guiana were born in the region, 9.3% were born in Metropolitan France, 3.0% were born in the French Caribbean departments and collectivities (Guadeloupe, Martinique, Saint Martin, Saint Barthélemy), and 30.2% were born in foreign countries (primarily Suriname, Brazil and Haiti). read more