Why did Flanders become a pars pro toto name for the Dutch speaking part of the Southern Netherlands (now Belgium), and not Brabant?

Answer 1

Belgium, in its national symbolism, is itself already based on the Duchy of Brabant, and thus Flanders is named after the County of Flanders. Flanders in the Middle Ages was embodied by East (Ghent) and West (Brugge) Flanders that together represented the County of Flanders (plus some portions of today's northern France).