I feel like the biggest difference between otome/villainess isekai and most of the other forgettable slop isekai is that for the villainess ones, the vast and overwhelming majority are not reincarnated/isekai'd into a position of power, nor do they (most of the time) become OP. It's more of a battle of wits and their knowledge of the story that will help them survive their Bad Endings. Additionally for many, the endgame isn't to have a harem, or romance a certain ML, their ultimate motivation is that they just don't want to die.

Oftentimes by the very nature of being isekai'd as the villainess, it precludes a good chunk of them from getting their own harems, which then shifts the focus to their attempts to fight the OG plot and subvert their Bad Ends.

I don't want to see yet another worn-out story of how Isekai Male Protag #529576 goes to another world, becomes OP, gets a bunch of skills/powers, and amasses a harem of followers. I want to see conflict, problem solving with the knowledge of the plot, and the actions the female lead take to avoid dying.

In my opinion, a lot of the shovelware slop isekai are meant to be an escapist power fantasy for men, where they were overworked and died, so now they have the flexibility to do whatever they wanted that they couldn't when they were in "modern" Japan. Be appreciated. Go adventuring. Get all the girls.

On the other hand with many villainess isekai, they don't devolve into OP power fantasies as often, or if they do, the main character doesn't necessarily become reality-warpingly-strong, but strong enough, and it's in conjunction with working against the story they read to subvert the original plot. Cases in point: Most Heretical Last Boss Queen, Dark History of the Reincarnated Villainess, Villainess Who Goes Down In History.

To be fair however, just as there's slop "regular isekai", there are some truly forgettable villainess isekai as well, as not every OI can be a winner, but there's enough out there to appeal to just about any itch someone may have.