@Psychronia I think that in some cases there's also another possible reason for the focus to be on the Villainess rather than the Heroine, though I think that this possibility likely arose as this genre progressed.
At first, most Villainess Otome isekai stories had the villainess be a bad person, but then the reincarnation led the plot in a different direction.
However, there's been since then more and more stories in which the "Villainess" isn't actually a bad person. Either she is framed, or she is only trying to keep away the Heroine without being malicious, and things end up with the Villainess in a bad role in the eyes of others.
After all, it makes sense considering that the Heroine is essentially seducing someone who already has a fiance, and that someone(prince in most cases) chooses to spend time wooing that Heroine even when he has a fiance.
And then, it's either the Heroine is the schemer or she's innocent(but still going after someone who already has a fiance) and other nobles who bullied the Heroine then blame the "Villainess". Moreover, in some cases, the Heroine even woo to her side many men(reverse harem), most of whom also have fiances.
And now, most Villainess Otome isekai stories follow this new format that focuses on the tragedy of the Villainess and how she rises afterward to make her life better. (Or how she changes events before the point in the game where she's be punished, and in those stories she is often still depicted as acting like the noble woman she was in the game, just also having hindsight and making different decisions.)
As such, in the evolution of this genre, we're currently in the stage where the Villainess is humanized rather than demonized as someone needing to be changed to the core to become a good person, for she is acknowledged as a genuine person from the get go rather than as a bad jealous person.
EDIT: Oh, and there's also how the "Villainess" is actually often someone more prepared and able to become a good Queen than a commoner or lowly baron daughter would be. Education to become Queen is also important. Furthermore, there's even more and more focus on how the prince who chose the Heroine is more incompetent precisely because he doesn't think in terms of the country but rather being more interesting in wooing a cute girl.