Now, there are two points here. One is that I don't think you can socially engineer the entire population of the Western World into exclusive veganism in a generation or even two; what you might well manage to do is to get people to eat vegetarian three or four days a week and cut their meat consumption. 90% of the population cutting their meat and milk consumption by 60% does more for the planet than 20% cutting their meat consumption by 100%. Why not stress cutting down over cutting out?
Two possible explanations - there is the cynical one which is that the proselytising section of the vegan movement is more interested in being pure and right than in actually saving the planet. And there is the very cynical explanation which is that the rich and powerful, who can actually do something about emissions, would rather shift the blame to ordinary people's diet and say 'it wasn't fast cars, and pointless wars, and endless plane flights - it was ignorant working class people eating Big Macs that killed the planet.' And they can then sod off to their mountain eyries and leave the rest of us to starve, choke and drown, with a clear conscience.