@lavendyer I have compilled a list of reasons for that:
- Commoner girl = gives a breath of fresh air to the prince since everyone around him are nobles
- Affectionate = more prone to physical displays of affection in general, the villainess is usually more cold or reserved
- Simple = since the Otome char hasn't learned noble manners she is less restrained in all of her actions so the prince founds her liberating
- Down to earth = since the Otome char has no ''power'' (political) she is usually humble and nice, the villainess is usually too proudful of her position
- New = the prince usually know their fiancee since childhood so a new girl brings new things to his world
- No Ambition = the Otome char usually doesnt care about the prince's power so the prince feels she can see his inner value
Of course this is only for real Otome MC but in most of this isekais where we follow either the villainess, a member of her group of a random NPC girl, the Protagonist is usually an asshole that knows the story too, so they are fully confident about the endgame, and they use everything i listed above to charm the targets.