Magic and electricity don't get along, because they are too much alike. That would push back your threshold a fair bit. Magic and certain kinds of chemistry don't get along, in that magical folks are more sensitive to or just react differently to many substances compared to nonmagical people. And when a lot of folks start thinking that magic isn't real, it nails the frame onto reality more firmly, so it's harder to pry loose.

It's not just a single thing, necessarily. All kinds of stuff can combine to suppress magic and/or kill off the people who have it.