CIG's Chris Roberts offers a lengthy explanation on Star Citizen's official website, complete with a list of critical bugs soon to be squashed.
It would be foolish to release an unstable build, even if pre-alpha for the sake of meeting an internal deadline. This is the power of the crowdfunding that made Star Citizen possible: a publisher would make us ship tomorrow regardless of the current build quality... but as you are all focused on quality rather than a financial return for shareholders we are able to take a few more days to deliver something that is stable.This is what can happen with an open development process, especially when we are sharing code and content long before one normally would in traditional development.
I know that's not the news you wanted to hear tonight. No-one would like to see the community get their hands on Arena Commander more than I would.
[Thanks Kommissar K!]