It’s more the case that the weight axis is common, and commonly understood, so the Google Fonts front end just leaves out any axis that isn’t Weight or Italic, I guess as a “UX” consideration. It’s unfortunate. The good news is, the Google Fonts API (the thing that serves fonts and CSS based on specific URLs) _does_ handle the other axes.

So, the Recursive minisite has a configuration tool (under the “Get Recursive” button) that makes it easy to get Google Fonts embed code for all axes.