Why didn't it occur to people in the 1800s that slavery was wrong?

slavery goes on today

https://www.antislavery.org/slavery-today/modern-slavery/

It still exists. Just in a different form. Human trafficking is a huge money maker. Just look at Olympic champion Sir Mo Farrah's story on how he became a British Citizen.

https://www.bbc.com/news/uk-62123886

Also all the sweat shops in Asia for the Western consumer to gobble up their products at a vastly increased cost to the buyer. These workers may be free to go home, but they have no rights or freedoms during their 14 hour shifts each day.

https://www.theworldcounts.com/challenges/consumption/clothing/sweatshop-facts

In fact there was a famous case in Britain a few years ago about sweat shops operating in England serving H&M and Boohoo and the like.
The owners were paying the immigrants minimum wage into bank accounts so that they look legit, but then took the money back off the "employees" who were forced to work in these awful places. It's probably still happening too.

It's the cost of fast fashion which most consumers just ignore.

Making money took precedence over everything. No matter how moral or immoral.

it is a shame still going on today just different