IMO people are still using rails because a bunch of orgs moved to it from Java years ago, or started with it after some successful hackathon. It's in wide demand because a large number of orgs mindlessly (or mindfully) chose it long ago. Basically all enterprise/large company apps I've seen have been in either Java or Rails. Some places are still using COBOL. Some places are still maintaining/starting codebases in COBOL. I assume that people will be able to make this same argument about NodeJS in ~2-5 years.
Also, the the point about micro-frameworks being bad being made in the comments because you eventually re-implement rails seems to be wrong or badly articulated -- that's exactly the point of using a smaller-yet-composable tool, you don't use any parts until you need them. Sinatra has also leaned into this by allowing pieces of rails to be loaded, and providing a path to transitioning to Rails proper.
I dislike rails because there's too much magic, and too many ways to do things -- it's precisely why I like micro frameworks. Also, the (request, response, next) middleware/handler paradigm is the most composable way to write server software I've ever seen.