I wonder if it's simply the fact that there really isn't anything driving them to get their kids vaccinated rather than a particular religious conviction. In Ontario, the old-order Mennonite and Amish groups have separate schooling for their kids and aren't integrated into the medical system here (not even being a part of our public health insurance system). Your family doctor and public health agency (through the schools) are the avenues the vast majority have to vaccination and so being apart from that, the old-order families would need to make a special effort to get vaccinated above and beyond what most people need to do.