@livingice Villainesses have become so popular for a few reasons:
1)It's a nice way to mix things up, rather than be the heroine.
2) The villainess can be more of a active character rather than a reactive character like a heroine (usually), making the story more interesting.
3) It can draw upon the readers resentment for the "blessed" as the heroine usually achieves happiness/love extremely easily and the villainess suffers.
4) It is the new trend (like isekai), so authors making new stories are likely to simply join in.
If we are talking about stories where the villainess is the MC:
1) The same reasons as before.
2) It provides a nice jumping off point for a isekai since many are familiar with otome games.
3) The character being the villainess makes for better conflict. Usually, for the heroine, the conflict is just the villainess bullying her, BBEG doing shit (who often is a love interest, kind of neutering the conflict with him by effect), and the love interests fighting over her. When it comes to the villainess, she has to contend with her often terrible reputation, fighting the heroine's love interests who are often uber powerful or will be uber powerful (though the love interests switch to the villainess' side often, making her basically the heroine), and juggling things that come with her position on the social hierarchy (as the villainess is usually above the heroine socially).
I must note that these are just some of my personal theories so you should take them with a grain of salt.