This is the result of a failure of public health to reach out to these religious communities in effective ways for decades.
Put another way if the overall population sees an average of 0.5 or 0.95 infections per case there’s zero chance of a huge outbreak. But odds of a case making it to a vulnerable population is wildly higher in the second case.
They never really stopped, it's been every few years since then: https://forward.com/news/417390/measles-is-hitting-ultra-ort...
The Amish are/were undervaccinated but it wasn't due to religious objections. It just seems uncommon in communities to see a dr, unless it's needed: https://www.pbs.org/newshour/health/ohio-amish-reconsider-va... It's also hard to get an official count (I've seen estimates below 20% vs almost 90% for non-amish communities in same state, but then you read stuff like this which suggests even the old older is above 80% https://publications.aap.org/pediatrics/article-abstract/128... ) since these communities are grandfathered into their own healthcare systems and often exempt from the normal federal welfare systems: https://www.ssa.gov/faqs/en/questions/KA-02411.html
I'm not sure about Mennonites. One of their communities writes about it and seems to suggest only 1 of the 40 or so communities is hardliners against vaccination. But I also note this is written in a really neutral way (could be to placate government, dunno): https://www.mennoniteusa.org/measles/
[1] https://www.cbc.ca/news/canada/calgary/alberta-mexican-menno...