I don’t think Cockroach cares about adoption. This is not meant to be a generally useful product in itself outside of their database. So the language was chosen mainly based on their familiarity with Go and its ability to integrate with their existing codebase.
I think their omission of major features such as transactions is more likely to limit adoption than the language choice, so language choice is kind of irrelevant from that perspective.
The integration with their existing codebase might benefit from an explanation. In Go, calls to C libraries are done using a compiler feature called cgo, and there's a performance penalty that the cockroachdb team measured at 171ns per call, plus you sometimes have to copy additional data around for memory safety[0]. So far as I know, there's no way to avoid this penalty that doesn't involve building a tool that does the same thing as cgo.
In this case, given that they already have a database written in Go, writing the backing kv store library in Go has a clear performance benefit of 171ns every operation. This is above and beyond just familiarity and easy integration, although those are also important.
I think their omission of major features such as transactions is more likely to limit adoption than the language choice, so language choice is kind of irrelevant from that perspective.