Many, if not most, environmental arguments tend to focus on the serious impacts of beef. Although we see a vegan diet as optimal, most people instead see chicken as being vastly superior to beef (and increasingly cheaper than beef). Any time someone replaces beef with chicken, many many more animals suffer.
Here is a great argument that actually helps chickens, from New Scientist (March 21, 2015, p. 44):
Such a switch [from chickens to plant-based substitutes] could make a difference to the environment: if we all swapped chicken for beans, for example, greenhouse gas emissions would be much lower. Chicken is responsible for 6.9 kg of greenhouse gases per kg of meat, compared with 2 kg for bean protein.
Cross-posted at the VegFund blog