By controling the platform, Google can leverage the ecosystem to attract and cherry-pick early adopters to the language.
Once the language becomes populat enough that the Enterprise starts to adopt it, Google will already be onto their next 'ground breaking' technology.
Once people settle on a perception they tend to hold on to it for life. One day soon people won't want to use Java. It'll be seen as "our Dad's language". Steve Yegge (ie now a Googler) said as much during his OSCON 2007 talk.
The value isn't in the language/platform, it's the people who adopt/build/use it.
Microsoft knows this, developers are what kept them afloat during a decade of stagnation in product development. Oracle thought they could buy their way in by acquiring Sun; except, while they were busy suing Google, Amazon sprinted ahead of the pack with their cloud services platform.
This is how the biggest players make long-term investments. Guarantee an abundant stream of top tier talent and success will follow.