Because these innovations and differentiated features can be kept proprietary, manufacturers and mobile operators are protected from the "viral infection" problem often associated with other licenses.
From Google's amusing FAQ on the Android platform, that's being marketed by the Open Handset Alliance as a "complete mobile phone software stack", that "includes everything a manufacturer or operator needs to build a mobile phone", and is "built on the open Linux Kernel".
Funny enough, the Linux kernel is only available under a license that's 'associated' with "viral infection".
I'll keep my pitchfork and torch stashed away, since Google can do no evil, but I'll point out that the web site for an open platform is somewhat pointless without the actual code, unless Google wants to have a go at enjoying all those benefits for keeping its innovation proprietary a little bit longer.
Would it have hurt to announce it when it was ready, i.e. in a week?