I've gone on about my reservations about older audiences, and maybe it could have done a bit better, but this is a pretty understandable scenario for Downton Abbey even if older auds were everywhere. Movies like Downton Abbey are super niche, since the only people who are interested in seeing the movie are already fans of the show. If you're not, what's the appeal? And while the first movie opened the way it did since it was the first instance of Downton Abbey content since the show ended, this one doesn't have any other hook, so only the super fans cared to come back. This is the same fall as something like Sex and the City 2 or even Pokémon 2000.
We're still doing this "OMG what if this streaming show hurts this movie" stuff? Okay then.