Think of it this way.

Most of us are not in an insular community because most Americans have multifaceted identities. Most Americans can cross over from different community to another very easily and get multiple different perspectives and information. 

If you are in an insular community, take for example Somalis, most of them mostly interact with only each other (for a variety of reasons, language and cultural barriers and unity makes them easier to relate to each other than outsiders). So if someone in the community starts raising concerns about something, its harder to dispel unless another member of the community disputes it. From the articles I've read about orthodox jewish anti-vaxx, the movement availed all kinds of anti-vaxx resources to orthodox jewish community members and from there it spread by word of mouth creating distrust of vaccine. Imagine if you speak only russian mostly and you come across a pamphlet in russian saying not to trust vaccines. They are trusting their 'community member' more so than doctors who may or may not be part of that russian community (this also leads to the whole affirmative action discussion about creating more doctors for minority communities). 

In a way, people within these insular communities were duped and were victims of rabid anti-vaccination movement and their pushers.