Vegans are promoting or have caused a monoculture of crops across the United States, meaning only relatively few crops are grown in a given area.
Carnists will claim that crops like broccoli are using up resources but not giving us many calories, which would mean that we should then only farm the one crop that gives the most calories per acre, a strategy that guarantees a monoculture.
Again, animal agriculture is much more resource intensive than plant agriculture. In addition, people aren't generally feeding animals obscure plants like kale. No one is feeding animals fruits of any kind, nor other distinctly human foods like tomatoes, olives, or cucumbers. As a result, animal agriculture relies on relatively few crops: Chickens are largely fed only corn and soy with some vitamin enrichment, and cows are generally fed grain and soy.
So it's actually kind of weird to claim that vegans are causing a monoculture of something like corn across the United States or soy in South America because those are the staples of farmed animals' diets.