When Kourou in French Guiana was selected as the spaceport's site in the mid-1960s (after agreeing to withdraw from its previous base in Algeria after independence there in 1962), it was the clear favourite out of 15 possible equatorial sites. (This was not counting France itself, of course.) This was because French Guiana a) is quite close to the equator (thereby facilitating launches), b) already had port and airport facilities (just waiting to be improved), c) has had political stability due to its being politically a part of France. In this scenario, the last part is butterflied away (being a part of Brazil), and after all, Belem had also been considered for the spaceport site but was eliminated as a choice largely because of political stability issues.
The next best choices for the siting of the spaceport - Darwin, Australia; Belem, Brazil; Tuamotu Archipelago, French Polynesia; and Trinidad - all had issues related either to political stability, vulnerability to hurricanes, or were simply too far away from Europe. Other choices included a couple of sites right around Guadeloupe (a French dependency in the Caribbean), Madagascar, the Marquesas (in French Polynesia), and Mauritania. Just look up the Guiana Space Centre's
website for more.