Well, initially, their investment in more runtimes was covering additional languages, not covering more versions of languages they already had. (IIRC, AE started with Python 2, added Java first, and then later Go and PHP.)
As that was happening, they gradually shifted focus from building out the old, fairly limited model of App Engine to building out other parts of the cloud platform, including (particularly relevant to future App Engine work) Compute Engine, to have a more robust cloud offering covering more use cases.
Then, they went back and returned to improving App Engine -- leveraging the now-broader cloud platform, and particularly Compute Engine -- shifting from the old model for new runtimes to the Flexible Environment (originally called "Managed VMs") model (built on Compute Engine, and which, in addition to including some additional Google-provided runtimes, means people can package up their own runtimes for App Engine).
Since doing so, they've released official Google runtimes for Python 3, Ruby (which people have been asking for longer than Python 3), and Node built on the Flexible Environment.