> C#'s speed advantage over JS among many other things would make C# the main language
Nobody cares about this, JS is plenty fast for LLM needs. If maximum performance was necessary, you're better off using Go because of fast compiler and better performance.
This has always been the case. The Java and C# ecosystems prioritise stability and scale. They wait for ideas to prove themselves in other languages like Erlang, Python, Go, Scala, and so on, and then adopt the successful ones. Last-mover advantage. That said, there are some caveats. Java is still missing value types, while C# has moved quickly with async/await rather than adopting models like goroutines or virtual threads, which can sometimes limit concurrency ergonomics for the developer.
> Nobody cares about this
And that was my point. The choice of using JS/TS for LLM stuff was made for us based on initial wave of SDK availabilities. Nothing to do with language merits.