Asymco:

Google today is faced with the prospect that not only the devices (which sit on its OS) but also services and apps on top would choose to remain on old versions of Android. There is nothing to stop them from doing just that. Unlike Microsoft, Google does not enforce licensing terms for Android. It is at the mercy of the value chain.

In other words Google shot themselves in the foot by not writing their licensing in a way that would allow them to force vendors to use the latest and greatest in a timely manner. Oops.