I am also curious but I agree we shouldn't judge if we don't know why. I know too much folic acid has been linked to increasing chances of certain cancers so if women feel they already have enough FA in their diet then it may be a personal choice not to take a supplement and not because they are risking their child's health.
In response to the op I didn't know I was pregnant until 5 weeks and hadn't been taken supplements. I'm not too worried as I eat alot of leafy green veg and cereals with folic acid in as well as the bread I have. Even if you were lacking folic acid it is still very unlikely you will have caused any harm to your baby. I started taking it once I found out 400mg a day but I stopped at 12w xxxxx