Why not Java? It's more powerful, mostly faster, has superb tooling, high quality libraries for just about everything, tons of very experienced developers, etc.
Java takes longer to build with and it's harder for startups that are working on product market fit. Once we figure things out, we can rewrite in Java, but before then it's premature optimisation in my book.
> Java takes longer to build with and it's harder for startups that are working on product market fit.
Longer or harder than using Golang? Unlikely.
Golang has some advantages over Java like a simpler deployment model, less runtime overhead, etc but overall for a fairly standard web application it makes much more sense to pick Java over Golang.
> but before then it's premature optimisation in my book.
Then so is using Golang. You have answered your own original question :). Keep using Ruby.
Particularly look at the orange bars in the graph "Programming Languages" on [1] which is based on data from AngelList.
1: http://codingvc.com/which-technologies-do-startups-use-an-ex...