As a vegetarian myself, I've thought about this a little bit.
I think it ultimately boils down to the fact that going vegan requires a lot more work from an individual. Avoiding meat might be a pain in the ass to implement at times, but the actual intellectual process is straightforward. You need to watch out for soup stocks, cheeses with rennet, and meat sauces basically. Everything else, at least in my experience, is obvious. Converting a recipe to vegetarian doesn't require too much thinking. A lot of foods are just innately vegetarian and won't be labelled as such: there aren't "vegetarian pancakes" or "vegetarian pies" out there — they're just expected to be vegetarian unless someone made a meat version. Only a small handful of pizzas will be labelled vegetarian even though most are or trivially can be made such. It's easier to find/adapt recipes that are vegetarian compatible.
Going vegan is just a full extra process. Eggs, milk, butter aren't visually obvious. Even bread isn't certain to be vegan-friendly. The ingredients being removed from a recipe cannot be simply removed, especially with baked goods, without risking the entire recipe becoming a disaster. If you take a cookie recipe and remove the eggs and butter, you're going to be disappointed; you need to find a recipe designed from the ground up to not use eggs or butter.
The extra restrictions on vegans mean they need to be much more specific about their foods than vegetarians.