Why do people think women can't be Vegan and pregnant at the same time?

My wife and I are Vegan. She lives on a high quality organic Vegan diet and eats extremely nourishing nutrient dense foods. Right now she's pregnant with our twins (a girl and a boy) and she has been having a very healthy pregnancy. She absolutely never gets sick and is a picture of perfect health and vitality. Yet people who don't live like us are always criticizing saying she should be eating milk/dairy and meat and that she is depriving the babies of "nourishment" and is a terrible mother. Like everyone seems to think they would make better parents. My wife is very educated and knows what she's doing. We think we'll be great parents and have the right to raise our children with any beliefs we choose. Why are people so unwilling to accept ways of living that are different from their own?