It's not a trend , it's just authors are just too lazy to come up with something new , so they just copy paste .
idk if this applies but usually knights are depicted as like upright chivalrous church boy gentlemen and imo it would be too easy of a choice to make, irl u look for someone polite and stable but for a story its probably more engaging to have the mc go for someone who creates conflict and makes the book dramatic like tje toxic cold duke of the north trope + highborn characters w status feel more appealing? probably somerhign about them being able to give mc all that they want
I think it's because of some, for the lack of better words, superiority complex (?). My thoughts is that those kind of authors will always favour the king more because the king possess the most power in the entire kingdom.
And many of the author might have like subconsciously self inserted them in the FL's shoes and obviously they want to have the most powerful ally by their side.
If they chose the knight then there still exists a woman with the higher/highest form of power aka the queen. Maybe they can't bear to let the FL be "weaker" in terms of all powers including political. Idk if I explained it correctly but hope you understood what I meant
Of course there are exceptions, but I've read life 2673632373637 historic romantasys, isekai or not, at this looks like a trend. Is it because their loyalty can be won without the romance? Does anyone have a solid theory here?