Bankrupt is a politically loaded term, but they have very high debt and taxes, political gridlock (it is very divided among French and Flemish linguistic lines, plus all the other traditional left/right polarization), and it is all but impossible to make reforms. IIRC there was no sitting government for 500 days at some point. It's also got all the classic problems of an aging population.

Belgium is a curious country that was formed via historical quirks around religion (many Flemish/Dutch speaking catholics not wanting to be part of protestant Netherlands, but that is a gross oversimplification and the history is very complex - read up on wikipedia if curious). Historically the Flemish were the poorer part of the country, but after deindustrialization the story flipped as most of the industry was in the French parts. The result is bitterness that holds the whole country back.