Google wants developers contributing to their projects, so they go where the developers go. The developers have moved onto GitHub and have effectively abandoned Google Code, therefore so has Google. Stubbornly maintaining their own service no one wants just breeds confusion and and a division of resources.
As the article says:
> To meet developers where they are, we ourselves migrated nearly a thousand of our own open source projects from Google Code to GitHub.
Nearly a thousand open-source Google projects have been migrated to GitHub already.
That all sounds weak to me. Google largely abandoned Google Code before all the developers fled to Github. One of the reasons people went to GitHub because it was clear that Google was no longer investing in Google Code. You're painting a picture where Google is just a passive entity that has no control over what developers do, it just meekly follows them around. That is hardly giving an impression that developers are a high priority to Google. If they were a high priority, Google would want them all using a Google service, and be willing to invest in that to keep them there, not cede them to third party.
As the article says:
> To meet developers where they are, we ourselves migrated nearly a thousand of our own open source projects from Google Code to GitHub.
Nearly a thousand open-source Google projects have been migrated to GitHub already.