My assumption is that the weight of including vegans socially is too much to bear for some. On the extreme end, you can't use store pasta because it was made in a facility that processes eggs and dairy. But even on the lighter end, social eating modification is pretty extreme: don't use butter when cooking, or eggs, or honey, etc. It's a lot easier to accommodate vegetarians than vegans.

Though I do have a vegan friend that always brings her own food to gatherings, and it does make things far easier.