Sometimes it's for religious reasons, sometimes the family are vegans and they want no animal products in their kids, some are worried about the mmr and the link to autism (if there really is one) and some don't want to cause of bad reactions on family members.
I got Trey all his vaccines including the chicken pox and flu, because of his asthma I need to be vigilant about him getting sick. I never had problems with any of the vaccines and there's nothing in my religion against shots, so trey has them all and my future child(ren??) will get them too
jmo