Largely because instead of having a socialist, liberal, Christian-dem, green and far-right party like other countries, we have two of each of those. Or had, anyway; right now, we have six major parties in Flanders, and four in Wallonia, used to be five and likely will return to five before too long. So even in federal elections, Flemings can only vote for Flemish parties, and same in Wallonia - there are no country-wide parties. And apart from the general policy differences - with Walloon parties in general being significantly more left-wing than their Flemish counterparts - there are the language disputes that essentially pit Flemings and Walloons against each other, with any party that tries to bridge the gaps accused of being a traitor to its community.

We should have realized a long time ago that we needed *some* kind of election in which Flemings could vote for Walloon parties and vice versa, in order to make such bigoted anti-other-language plays self-defeating. But we didn't, and any attempt to introduce such a thing now is likely to fail as the most nationalistic parties will fight it tooth and nail.