I think it's a subtle manifestation of ageism in startup culture. From my experience, most Java developers started with the language, community, and ecosystem when it was young, vibrant, and new. Those programmers (like me) are now in their 40s. A lot of them now work in beige cubicles for larger, more family-friendly firms (many by choice, so perhaps "ageism" is unfair). Early in your career, you see opportunity in new ecosystems. You see other people like you taking risks and experimenting with shiny new stuff and all the hype around it (hey look, now I'm the one being ageist!). The more mature you get, the more fads come and go, and some of us are a lot more cynical about whatever johnny-come-lately is being blogged about.
Rails, Django, even PHP, and newer technologies like Elixir and Golang - they're all new and fun. Younger people have the time, risk tolerance, and predilection to hype for the latest fads. There are opportunities to "make your mark" as an expert by contributing to FOSS projects and writing the next Rails in the next new language.
And startups like young people. Google Silicon Valley Ageism. Read pg's tweets (https://mobile.twitter.com/paulg/status/655490006216146944). And young people like startups - they still see glory in working 60 hour weeks and sleeping at their desks.
I recognize that there are obviously lots of exceptions to all of this, and I'm probably being a bit dangerous speaking in generalities with my personal experience.
So tying back to the original question: Java and .Net were hip in the 90s and early 00s. In those eras, the people who worked with those exciting new architectures were the people in their twenties in those eras. They're older now, and startups tend to attract younger employees (call it ageism or call it "startups aren't attractive to older people." It happens). You don't see a lot of twentysomethings looking at the JVM and thinking there's a lot of opportunity there to make their mark as an expert. It's not the latest fad to be posted on Hacker News every other day. You don't see posts titled "A ___, but written in Kotlin"on what is (ostensibly) a site about startups. You see a lot of "A ___, but written in Go."
Having said that, I'm a forty-something Rails dev who's learning Elixir as we speak. So perhaps I have no idea what I'm talking about. ^_^