- Scala: only really took off with Spark (had Akka before but wasn't really that popular)
- Python: might've died if not for numpy/pandas/ ML
- Ruby: Rails
- Typescript: V8/Node.js / javascript on the server-side
- Go: Google, Kubernetes (and even so it's questionable how popular it _really_ is)
- Swift: Apple, iOS apps (to a somewhat similar extent: Kotlin, for Android development).
- C#: I can't really name the "killer app", but... Microsoft. (to be fair the "killer app" itself might be the whole MS ecosystem integration)
There are a lot of decent languages (Clojure, D) that never got popular, despite their merits. Even Kotlin mentioned above is arguably a much better choice than Java on the server side. But it's not used that much on the server, even though it actually has the answer to "what problem does it solve" (easy way to write async code in JVM, which is not a small thing)