I doubt this is why Go was chosen over Ruby, it would be fast enough for this problem. It probably has more to do with distributing the software.

Distributing programs in written in Go is _much_ easier that distributing them in Ruby, i.e. cross compile and ship the binary, but distribution of Ruby programs is hard.

Too much time is wasted on 'omnibus' projects to distribute Ruby programs, just look at Chef or Sensu or Mitchell's own Vagrant installers project:

https://github.com/mitchellh/vagrant-installers

(I don't mean to say those projects were a waste of time, but that it seems like this should be a 'solved' problem).