The reason why Belgium is mainly bilingual has a historical reason.
In the 19th and early 20th century Wallonia was the most industrialised and wealthiest region and so French (it’s language) in Belgium dominant. Meaning that the upperclass in Flanders spoke (only) French too. The same for teaching at universities there. Flemish was more for farmers and the working class.
During the course of the 20th century, especially after WW2 Flanders became more industrialised and so more wealthy, while that of Wallonia declined. Flanders economy became dominant. With this the emancipation of Flemish became more and more important. Causing a serious language conflict, still having it’s influence till today.
If you know more about Belgium’s history than you know how tough and painfull this struggle was and still is. Especially defining the location of the language border caused many riots and political upheavel in the past. Nowadays Belgium has a gouvernement for Flanders and one as well for Wallonia, each with it’s own language. Brussels is an exception, has it’s own gouvernment too, but is bilingual, Having political reperesentatives from Flanders as well as Wallonia. To keep this working is a serious balancing act.
There is also a relativily small, but official community in the eastern part of Belgium close to the German border spreaking German. So adding English as an official language will make everything more difficult in an already mindbogling complicated situation. However Brussels being the capital of the EU and home to NATO headquarters it's obvious that English is much spoken there too.