Because I am being realistic. Ruby, C, Java etc have had decades to build a comprehensive set of developer libraries. Go hasn't.
It's a "cute" platform that I am sure will beat Ruby in a few benchmarks. But I don't see any reason why anyone would seriously consider it for a real world app.
Ruby has a sprawling ecosystem of libraries largely drawn into being by Rails (itself a sprawl) and by design decisions in Ruby's core and stdlib that complicate things and need further code to work around. Threads and fibers and events, oh my. Each workaround has its own ecosystem. You could even consider things like rack and unicorn workarounds for the weak implementations in stdlib.
I don't think Ruby's libraries honestly provide a whole lot better coverage than Go. The stdlib is very well considered. It's fast, it scales, it's thread safe in the appropriate places. It's not a toy implementation. What it doesn't provide, it provides hooks for. Third party libs cover all the major bases. And unlike Ruby, writing interfacing code in Go is a snip. I don't see libraries as a downside.
(With one exception. Why oh why did they not include a bigdecimal? Argh, a ratio type is not an adequate replacement. Try rounding a ratio some time. Blarg.)
Well for one Go beats Ruby in every benchmark that I am aware of, and generally by quite some distance.
Also realistically 60-70% of all libraries created for other languages are bitrotted to junk or pointlessness by this point.
If you can't see why an app that fits within the worked problemspace of Go (and even with the current libraries plenty of things are wholly doable in Go) and will require much less server resources might be a good fit, then you shouldn't be making those decisions, because that is the thought pattern of a zealot.
Look Ruby is well supported, established, and proven, but it is also slow. For a lot of things that is fine, but if you are dealing with something that requires a lot of serverside work on low margins it will kill you dead. Horses for courses.
The point is if you need the performance, C is the more natural choice for a lot of it at this point. I looked through one of the Go tutorials a while back, and it was WTF after WTF. If it works for you, fine, but Go has a long way to go to be interesting for a lot of us.
Because in reality, people don't make decisions based on how many duplicate, unmaintained, broken libraries exist for language X. They make them based on "does language X have the libraries I actually need". Oh, json, DB_of_choice, and web framework exist? Gee, I guess 90%+ of people are covered right there. Virtually every language, even as seldom used as say, ocaml, have the libraries that 90% of people actually use. Libraries aren't an issue for the majority of tasks people are doing.
