mostly because they don't understand.
most people have very little understanding of health and nutrition. my husband is an omnivore but eats very healthy. he doesn't eat fast food or junk food at all. People make comments to him all the time--i could never be that way, you eat too healthy, you need to loosen up and have fun with your food, etc--the same things that i get as a veg*n.
part of it is a sort of guilt. people want to fit in. they know that they're doing something not-so-great (i think nearly everyone realizes that fast food is not healthy for you) and when you're not doing it, it makes it glaringly obvious that they're doing something 'unhealthy' or 'bad.' they are self-judging, and in some cases, assuming that you're judging them, so they're 'beating you to the punch' by mentioning their unhealthiness first, and trying to focus on yours.
at least, that's the best i can figure.