I wonder if it's because of the whole HSH vs. HRH thing? HSHs are counted as lower than HRHs, but I thought exceptions were made for when HSHs are Heads of State, like Albert is?

In any event, I think Albert and Charlene attending is logical. He really doesn't have anyone he could send to represent him--although I did get a really good giggle imagining Jacques, and maybe Gabriella too, showing up at the Abbey with their nanny in tow!