Because Belgium is formed in 1830 by different provinces of
Southern Netherlands. And in the most southern ones people speak
French and formed together Wallonia. The northern part, these days
known as Flanders, is formed by West and East Flanders, Brabant,
Antwerp and Belgian Limburg.
Historically Flanders was the part in the west (the current
provences West and East flanders of Belgium) + Zeeuws Vlaanderen
(South-West of current Netherlands) + French-Flanders (north-west
of France).