So the thing people miss about this one is people who live out of the reduced/sale section. While at full price a vegan diet is cheaper (though requires a bit more prep time, not much more though microwave steamers are a miracle) Meat is much more calorie dense and can end up being as much as 80-90% off just before it turns, vegetables on the other hand never go on sale. In this circumstance meat is cheaper.

More regionally some of the foods in a vegan diet that make up for protein can be more expensive than you might be used to. Sure beans are universally cheap and there's some nice varieties (I like kidney and butter beans a lot) but chickpeas, nuts and really all of the non-bean alternatives are actually pretty expensive in some places (e.g. where I live).

That said I admit to being one of these people who could maybe drop meat (I only get it when its on sale/reduction at this point) but couldn't live without cheese and eggs. iirc chickens are the lowest carbon livestock but I await a good cheese alternative or non-dairy cheese.