America isn't the only country where slavery existed, and both slavery and animal welfare can be tackled in a region without having to change the entire world at the exact same moment.

If enough people in, for example, the US felt as strongly about not eating animals as they do/did about not having human slaves, they could have another "quaint" civil war (or maybe a political argument that could achieve the same thing without bloodshed) and get to a point where it would be illegal to eat animals just as it's illegal to have slaves.

(I'm neither American, nor a vegetarian, FWIW.)