It was a long gradual process that didn't start with isekai; it has been decades of gradually accelerated industrial pressure to pander to the biggest spending otakus in Japan (and weebs in the west) because they bought the merch.
The pandering used to be pantyshots during an otherwise passable fantasy adventure focused on the adventure. Over time the focus increasingly became pandering to the audience, not just in horny/creepy stuff but also in "your video game skillz allow you to metagame and conquer this fantasy world" powerwanks.
The escapism and power fantasy intrinsic to the genre combined with modern anime being designed to appeal to the type of otaku who'll spend hundreds on merch or dvds (or $40k on a statue of their waifu). For these types there's an appeal in a girl who literally can't leave them, hence slavery. Harems are partly wish-fulfilment, partially marketing: 5 archetypal waifus will appeal to more otaku than 1 archetypal waifu.
We started off with Magic Knight Rayearth, Escaflowne and Fushigi Yuugi
Before that we had the Neverending Story; it hits every checkmark for the isekai genre except the modern expectation of "video game boy wins at everything and gets the sex trophies."
There are a few proto examples from western Baentrash fantasy like The Radiant Knight (do not read) that got picked up where the power fantasy (that was already being deconstructed in novels like A wizard in spite of himself and The man who came too early.) was mixed with dumb horniness.
As the chan culture picked up and became more right wing and misogynistic both here and in Japan, this went from moderately problematic to a death spiral dropping all the cool social history and cultural/psychological examination parts of portal fantasy in favour of having a little world that you get to do free war crimes to but it's good actually.
How did isekai end up becoming the slave harem genre, anyway?
It was a long gradual process that didn't start with isekai; it has been decades of gradually accelerated industrial pressure to pander to the biggest spending otakus in Japan (and weebs in the west) because they bought the merch.
The pandering used to be pantyshots during an otherwise passable fantasy adventure focused on the adventure. Over time the focus increasingly became pandering to the audience, not just in horny/creepy stuff but also in "your video game skillz allow you to metagame and conquer this fantasy world" powerwanks.
The escapism and power fantasy intrinsic to the genre combined with modern anime being designed to appeal to the type of otaku who'll spend hundreds on merch or dvds (or $40k on a statue of their waifu). For these types there's an appeal in a girl who literally can't leave them, hence slavery. Harems are partly wish-fulfilment, partially marketing: 5 archetypal waifus will appeal to more otaku than 1 archetypal waifu.
We started off with Magic Knight Rayearth, Escaflowne and Fushigi Yuugi too how did everything go so wrong
(answer is capitalism, duh)
Ok Fushigi Yuugi could be pretty problematic at times but still!
Before that we had the Neverending Story; it hits every checkmark for the isekai genre except the modern expectation of "video game boy wins at everything and gets the sex trophies."
Narnia, The Fionavar Tapestry, and of course the ur example of Lest Darkness Fall which started the power fantasy creep.
A Connecticut Yankee in King Arthur's Court was pretty based even if somewhat power fantasy too.
Also strangely Marxist in it's ultimate conclusion.
Also ultrabased quote about two Reigns of Terror.
Alice in Wonderland, too
That one did have some sus background energy because of the author's own preoccupation with creeping on kids.
There are a few proto examples from western Baentrash fantasy like The Radiant Knight (do not read) that got picked up where the power fantasy (that was already being deconstructed in novels like A wizard in spite of himself and The man who came too early.) was mixed with dumb horniness.
As the chan culture picked up and became more right wing and misogynistic both here and in Japan, this went from moderately problematic to a death spiral dropping all the cool social history and cultural/psychological examination parts of portal fantasy in favour of having a little world that you get to do free war crimes to but it's good actually.