Speculation: Google Wave is implemented with heavy use of Google's internal libraries.
Wasn't google wave an HTML5 app that used google web toolkit? What other libraries would there be?
The libraries and infrastructure that power the serverside parts?
The backend.
BigTable.
I strongly suspect BigTable is one of the primary reasons they don't want to open source the current release seeing as they mentioned using a Mongo DB backend.
Thanks for asking this question.
Not speculation. Stated by Google employees on the Protocol lists explicitly last year.
I imagine that the current codebase is tightly coupled to their internal infrastructure. They're writing the stand-alone version for the first time.