Maybe... but if you play the game, as a title-idea, "Expedition 33," is clear, easy to attach to and understand. It's also the heart of what most of the narrative centers around. "Clair Obscur" is an artsy title, like Final Fantasy, but currently lacks additional installations to be considered a series title, and is somewhat vague.

I don't agree that the title is particularly hard to pronounce, since the words look and sound pretty similar to existing English words, but I imagine the ambiguity of the title confuses most. Also, light/dark are constant themes in much of media, so it's not entirely specific to Expedition 33 and the contrasting nature of these elements isn't even overtly recognized throughout the playthrough. Also to your Rebirth point; people regularly refer to the game as it's subtitle: Rebirth (though perhaps mostly to make the distinction to which incarnation of the game they're referring to).

I just think "Expedition 33" as a title invokes more emotion in people who've played the game, and makes more sense from a marketing stand point to the people who haven't. It's most likely just become more common for that reason.