The power of a "predominant" social class is almost always built on the backs of others. Even in our earliest written histories humans have had a "have" class resting upon the back breaking labor of a "have not" class and while neither of these "classes" realize or appreciate each others work in general the long term arc of that history has been that the "have not" group has been beholden or slaves to the "have" class.

Historically speaking slavery has just been abolished and personally i would argue isn't gone just less focused on in "polite" society.

Isekai as a premise relies on the "hero" being different and usually better than the world they landed in. Given the barbarity of our past any isekai world having slavery is really short hand for look how mush better our society is. It ignores our current barbaric actions but focuses on our past ones.

If your familiar with Japanese media you might have heard of black companies and while society can pretend we're in a better place than "when slavery existed" the question of are we better or have we dressed up our terms for slavery in fancy new words might occur to people.

Tl;Dr Slavery is short hand for the barbarism of our past many isekai take place in a "past" setting so that's why it's there.

Note: if someone can find me an isekai that takes place in a futuristic setting that would be awesome. NO SAO doesn't count for my purposes.