The BRF plays by its own rules and always has re the Continental RFs, even as far back as Queen Victoria. It's really hard to say why, but it's certainly easy to speculate.
With Liz & Phil, you'd think it's due to age, but look at 'em go all over the place still - so if there's something they really want to attend, they'll show up... but they didn't. To be fair, Liz doesn't usually attend these kind of events, but one would think that Phil, due to his blood ties, would. Maybe he has no patience for Marge's cray-cray.
Chuck doesn't seem to have close connections to them, though he certainly got to know the DRF through the years; maybe he won't go because of the snub of having John Donaldson (Mary's dad, of all people!!) do a meet and greet with them on their last visit - a snub to Chuck, to Cams, or both? But, IMO, a snub it was, and I'd probably feel the same way.
The Lazytons aren't much into shindigs like this, plus I don't think Wm has any real connection to the DRF other than that useless visit a few years ago. Plus, you're getting into whether they're too junior for Marge - send the heir to the heir and that's not good enough for Her Delusional Majesty.
Continental events are not Anne's or Andrew's cup o' tea. 'Nuff said there.
The only others with any connection to the Continentals are the Wessexes, but for Marge's self-celebration, they were certainly too low in the pecking order to be permitted to attend, probably for fear of offending La CRAZY Reine.
The queen's cousins (Kents, Gloucesters) are again too junior.
Maybe the Firm at large knows that Marge is loony tunes and wants nothing to do with it?
Perhaps someone can correct me, but doesn't Marge slip in and out of London and do private visits then? I know that Sofia of Spain does. Someone can refresh my memory. So maybe formally they're not great on attending, but have good personal relationships away from the camera...?