Because it's a pre-release. I want everyone to be excited for the release, and not be let down by the still-a-bit-buggy Beta. I didn't want to pre-release anything originally, but I figured I had to to speed up the bug finding process. That's why it became a Closed Beta.
The starting excitement of the release of anything is a huge boost. I know some games that were playable years before release, and they had a super slow and unexciting start because everyone already was sick of playing these unfinished versions. Secrecy is some kind of psychological marketing strategy