Probably because if it's opened to the general public with bugs and unfinished features, the developers will be so inundated with answering the same questions over and over about known issues that they will never be able to finish the darn thing. It's easier to keep a small group "in the know" on known issues. Probably also so it wont crash anyone's board until it is thought to be 'stable'.
We will serve no wine until it's time. Paul Masson
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