Because Google has a vested interest in getting their name on your platform, and their services to users via your platform.

Right now all the vendors run a tight gauntlet with Google, balancing pushing them under the covers for the OS that they've delivered, but also keeping them at enough of a profile that they continue to invest and develop the platform. Push them under enough and they'll quit. Let them take over and your identity as a brand is subdued by theirs.

With MeeGo, you attain compliance with a certain software base but the branding is all yours. Contribute to the core OS development, which is coordinated by a singularly focused 3rd party, in the open, with no extended leverage possible by one party, and you end up with something much more useful to the marketing robots who are charged with selling your stuff.

None of the hardware vendors (Samsung, Motorola, Nokia) want to play pure hardware vendor as that's almost a commodity market. They want to play the role of services vendor, which is hard to do when your OS vendor also wants to provide it.