Economics get in the way, too. As demand for beef for ethical or environmental reasons go down, that just reduces the optimal price for beef, which means those who would have bought more but didn't because of the price can now buy more of it.
Not saying this is a reason to not reduce your beef consumption, just that the system is so complex and the results are delayed (farmers need to guess how many calves are worth raising for beef/dairy vs just turning to veal long before any of that meat will make it to market, and those calves are getting made either way because that's how the dairy industry works: milk requires pregnancy).