Maybe it's to focus more on the main plot or other key characters. Sometimes, background characters don't get as much spotlight even if they seem interesting.
Maybe the focus of the story is on other elements or characters, so the duke takes a backseat.
Maybe Duke is doing it to achieve a certain goal or to protect someone or something important. There could be multiple reasons behind his actions that are yet to be revealed in the manga.
Maybe Duke has his own motivations or is driven by some internal conflict that we haven't fully understood yet.
I'm reading it because it has an interesting plot and engaging characters.
There could be a bunch of reasons. Maybe their social statuses don't match, or there's a family feud going on. Or perhaps there's a prophecy preventing the marriage.
Maybe Duke is driven by some hidden motives or past experiences that haven't been revealed yet.
Maybe it was part of the plot's development or to fulfill some important mission.
It might be that the story has a plot twist where their love is forbidden for mysterious reasons. Or perhaps the duke is already committed to someone else or has a hidden agenda that makes marriage impossible.
It might be that the Duke is responding to a perceived threat or trying to achieve a greater goal. We'll have to keep reading to really understand.
Raeliana ended up at the Duke's possibly because of some hidden conspiracy or a twist of destiny. The story might have woven a web of circumstances that guided her there.