Over time, religion became less salient, leaving language as the great divide. It's natural to sympathize with other people with whom you converse more than with other people with whom you can't as readily interchange thoughts. It's also easier to monitor them to make sure they aren't cheating you. The rise of NATO and the European Union has made the sheer size of a country ever less important for warfare or trade. So, states increasingly exist in Europe today less as part of a great game to accumulate the most military-industrial might to conquer other states, but mostly as affirmations of nationhood and as a means to redistribute wealth, both to interests and to the pockets of the leaders of interests.
In the past, both the aristocrats and the leading coal and iron regions of Belgium were French-speaking, so they had most of the money. Over time, however, the Flemish have become more productive, and resent having the wealth they earn taxed away and, net, given to Walloons. Both sides rightfully resent the corrupt rake-off by politicians, which is unusually high for northern Europe. My guess is that Belgium is not only unsurprisingly more corrupt than the Netherlands to the north but also more corrupt than France to the south, although I haven't looked into this for years.
Mixed ethnicity democracies tend to be crooked for what might be called the Lee Kwan Yew-FDR reason: You can't afford to vote out a corrupt SOB of your own group because while he might be an SOB, he's your SOB and — at an admittedly high cost — he protects you against the other guys' SOBs.
Belgium has been haltingly devolving toward a decentralized Switzerland model, but it might make more sense to just split the country into two countries along language lines with perhaps Brussels becoming the Vatican City of the EU.
But there's tremendous resistance to this sensible solution among the Euro-elites. The NYT says, reflecting the unthinking elite consensus:
"Europe as a whole may be busy papering over its differences, burying cultural disparities and centuries of feuding. But not Belgium. It seems headed the other way."In reality, the splitting up of Belgium would be a triumph for the European Union, showing that countries don't need to be big in Europe anymore to avoid being trampled on the battlefield or isolated economically, and can now afford to reduce themselves to sizes more congenial to honest, effective self-rule and national affirmations. But, that's too sophisticated of an idea for Euro-elites. They've been trumpeting themselves for 60 years as "burying cultural disparities and centuries of feuding," so they feel they can't afford to let Belgium, the proto-EU, break up, not matter how much better it would be for good government.