The earth runs on cycles and everything on it does also pretty much. When I was a kid we had lots of them, then they disappeared and now they are back here also. We had a cabin up north on lake Erie when I was a kid, and they were there no problem. we of course did the whole catching them in jars things constantly.
We were on vacation, last month, in the Smokey Mountains. They have warnings in the park now that it is illegal to collect lightning bugs. There are thousands and thousands up there and the kids cannot do what we did for years.
It's illegal to pick plants or bother any wildlife in the national park. Not so in your backyard
I moved to lower Alabama about 5 years ago and there weren't any fireflies, last year we saw a few and this year a few more.