I've figured out what I did wrong in retrospect most info was in the faq, but for some reason I didn't get it the first time.
When trying again, I wrote down what is needed in each phase, and updated the document as we learned the ropes. This document can be found here for reference.
First, the question contains a false assumption, as is clearly explained in @animuson's answer. Closure and removal/deletion are different things.
How does the system work?
The steps are listed in the faq under "Once I propose a site, what happens next?". However, it is not very clearly stated in one place what the criteria are for moving from one stage to the other. These criteria are exposed once you reach the next stage. It's great to know in advance, so you can tell your community what is expected of them. Many enthusiastic experts don't enjoy jumping hoops in area51 (although it goes without saying these hoops exist for a good reason).
So what are the promotion criteria:
- max 3 days
- at least 5 followers
- at least 5 questions
1b. promotion to commitment
- 60 followers
- at least 40 questions with 10 upvotes
- activity at least every 7 days
- people 'signing' the proposal. The faq says: "Users are asked to commit to participate in the site to assure that the site will have enough participation — we don't want to create ghost towns." This doesn't specify how many followers should sign, I'll update this answer once I find that info.
3. private beta
- at most 5 weeks
- further requirements unquantified in faq: "a healthy volume of high-quality questions and answers within up to 5 weeks, it will be deleted"
4. public beta
faq: "at least 1000 open questions, 70% of which have at least one upvoted answer"
The two subfases of 'Definition' are confusing, because at first it says 5 followers, 5 questions, and THEN suddenly that number jumps by a factor of 12. It is very helpful to inform your community up-front about all the hoops that need jumping before they can enjoy the real deal.