Given the opposition of the Flemish nationalist movement to the Belgium monarchy and Belgium generally, this is a historic moment. Certainly the political class in Belgium realises it is desperately short of options. The Cordon Sanitaire imposed on Vlaams Blok and its successor Vlaams Belang did not enjoy universal support even among politicians and media.
The Flemish Movement has long been divided over how it approaches the Belgian state. The moderate current typified by Volksunie and now N-VA appears prepared to pragmatically accept federalism. Vlaams Belang, representing the radical current, wanted nothing less than a Flemish Republic.
As head of state, King Philippe is required to be politically neutral. He cannot oppose the inclusion of any party in a coalition government. The survival of the Belgian monarchy, indeed the Belgian state, rests entirely upon the goodwill of the people - especially in Flanders.
BTW, the Francophone Socialists have NO moral ground to criticise anything. They've kept Wallonia and Brussels beholden to them for decades with terrible results.