Why does Puppet have its own language? Why not use XML or YAML as the configuration format? Why not use Ruby as the input language?

The language used for manifests is ultimately Puppet’s human interface, and XML and YAML, being data formats developed around the processing capabilities of computers, are horrible human interfaces. While some people are comfortable reading and writing them, this is the reason why we use web browsers instead of just reading the HTML directly. Also, using XML or YAML would limit any assurance that the interface was declarative — one process might treat an XML configuration differently from another.