It's plenty easy to find vegan junk food. Pretty much any potato chip that doesn't have the name of a dairy product featured prominently on the label is going to be vegan. Go to most natural food stores, including Whole Foods, and you'll find far more shelf space being devoted to processed and junk foods than food that's actually healthy. Most of it with labels that work very hard to imply that sugar only has calories if it's made from something other than sugar cane.

That said, you're not likely to find the vegan equivalent of Twinkies at your local bodega, and that is worth something. But I'm guessing that a bigger factor is that choosing to go vegan or low-carb or whatever is tantamount to deciding to join a food culture that places a much higher value on making better food choices, and there's nothing quite like peer pressure for changing one's behavior.