I answered a similar question here. Yours has a rather simple answer: French would be the dominant language of Belgium until WWI, and King Leopold ( who ruthlessly ran the Congo as his personal enterprise and spoke French) died in 1909.

At one time the Flemish had to listen to French speaking King Baudoin deliver a speech in painfully accented Dutch, and people in Wallonia would never bother to learn it. That has very much changed: the newer generation of Belgian royalty is quite bilingual, and as the economy has become much stronger in Flanders than in Wallonia many more Walloons are as well.