Many of the core team members of Rails have long moved on to other technologies.
Companies don't see Rails as something that will give them an edge anymore. Yes, it is quick to develop a prototype in, yet the ongoing performance optimizing burden and the mess it becomes at large scale are not worth it anymore.
Competition has been catching up, and many modern frameworks have taken the best out of Rails, while ditching the not-so-good parts.
The job market for Rails devs seems to be pretty dry. Many Rails devs I know, including myself, do not get even 10% of Rails job offers anymore, as it used to be until a couple of years ago.
Ruby, as a language, is completely stuck and stale, and has been for a long time now. There is no real innovation, the single-threadedness and the GIL are just not good enough in 2017. Ruby 3 has been announced for 2020, and if you ask me, what is promised in terms of features is rather too little, too late.
Rails, as a framework, has been steadily releasing improvements and new features, yet those are either not that relevant, or poorly implemented (e.g ActionCable). The framework's creator, DHH, has been very opinionated about the strength of Rails monolithic approach. To me, this is not where the industry is heading (like it or not) and Rails is going to have a tough time keeping up with the latest software architecture trends like microservices (again, like it or not, microservices are here and they solve real problems).
Rails will be there for a long, long time, no doubt about it. But for many of us, it's just not that sharp knife it used to be anymore. Better accept it sooner than later.