Belgian society at the time was mostly French speaking. There are several reasons for that: simple fashion (French was used in most European courts and our Saxe-Coburg king didn't make an exception), Brussel being French-speaking, and (mostly) the fact that the francophone south had all the coal and steel industries. Wallonia is arguably the second area after Britain to have gone through industrial revolution. On the other hand, Flanders were mostly rural and (I regret to say that as a Walloon myself) Dutch and their dialects were looked down by elites at that time. And it was usually those elite who profited the most from colonization. I remember elders saying that quite a lot of xenophobia was to be expected towards northerners if they moved to the south. On the other hand, money flowed northbound. It's only after WW2 and coal mines depletion that Flemish nationalism came to birth, leading to sometime violent conflicts, like the Royal affair, the exclusion of french-speaking people from Leuven universities, the Fourrons,... The linguistic power switched side as the south entered a transitional phase while the north became service-oriented.