We are using most of grown crops as feed in meat production. So we don't have to increase production to keep up with population growth, the other option is to cut down on conspicuous consumption in what we eat.
I think a lot of it if force of habit combined with the externalities of meat production not being reflected in the price of meat, making it cheaper to buy than it is in terms of overall cost. Meat consumption is a cultural element as well as a dietary one.
Increases in meat consumption are happening motsly in the developing world, and it's largely a status thing.
In the west it's a little more complicated since it's embedded in the food culture, and has a big systemic component (subsidies), but having a lot of meat at the table is traditionally considered a sign of prosperity here as well.
