Personal opinion, I'm sure many will disagree, but I've never been a fan of Ruby. It tends to be slow and, at least last time I used Puppet, there was a lot of dependency issues (from downloading them, to ensuring you run the right version of Ruby among other things). Whereas with Go downloading a single binary takes a whole lot of headaches away. Sure concurrency/performance won't matter much in the case of Puppet like software but depending on a single binary vs lots of dependency is a huge bonus.