All objected against a wedding with a mere baroness? Or they had other reasons to stay away. If it was the first one, the two youngest couple were a bit hypocritical indeed, as they had no problem to attend weddings of their own siblings(-in-law) to commoners.
The Belgian royal house requires only the consent of the monarch, so there is no issue of inequality in Belgium. If Philippe and Laurent had been Habsburgs, there would have been cause for Lorenz and Astrid to avoid the nuptials, but they aren't and Habsburg marriage requirements don't apply to them.