Because it's an article for non-graph-theorists. People tend to follow sentences involving familiar categories, than "nodes of H" and "nodes of G".

And speaking as a graph theorist. When you've got three graphs like this, and you want to unambiguously refer to nodes of each, leaving things perfectly general can get quite tedious and torture the language. For example, sometimes it's just nicer to just say "red nodes" and "blue nodes" where any colors would suffice.