A purity pledge is very typical of cults. It's both a filter and an enforcement mechanism.
This may not apply to Elm. But I imagine it can feel easier and more rewarding to manage a community that's more like a cult than a typical free-for-all open source project.
I think it’s probably harder and less rewarding to manage a community where you’re constantly taking flak for a technical decision people don’t like (and which those people generally don’t engage with the pros and cons of said decision!)
This may not apply to Elm. But I imagine it can feel easier and more rewarding to manage a community that's more like a cult than a typical free-for-all open source project.