Why does the third generation of Greek immigrants in Belgium use only French, while their counterparts in Germany speak excellent Greek?

Really, the question boils down to, why are Greeks in Belgium assimilating faster than Greeks in Germany.

At a guess, critical mass: lots more Greeks in Germany, so much more community life, much more community use of Greek.

I don’t know enough to speculate further, and I invite others to. Other factors could include:

• Demographics of the migrant groups. If, say, the German Greeks are all factory workers, and the Belgian Greeks are all European Union functionaries, then the German Greeks will have more cultural, social, and ideological blockers to assimilation, and the Belgian Greeks will have fewer. (I have no idea if that is the case.)
• Attitudes of the host population. If, say, Belgians all bed Greeks as a competitive sport, whereas Germans avoid Greeks in the street, there will be less assimilation on the German side. (I’m reasonably sure that’s not the case. But Germany did assume that Greek migrants, like Turkish migrants were all guest workers who were going to go away, and to everyone’s surprise they never did. If the assumption was not in place in Belgium, there would have been less resistance to assimilation.)