From Graduation, site closure, and a clearer outlook on the health of SE sites:
When a site starts to consistently receive 10 questions/day, we’ll consider it for graduation.
I've italicized two important words. Just hitting 10 questions a day isn't sufficient; we are looking for sites that sustain that rate. Also, there is an internal review process the community team uses to determine if a site is ready to graduate. (I don't see any reason for Writing to not graduate, but I don't make that decision on my own.)
As I've said elsewhere on the site:
Our long term goal has been to make sure that sites have the features they need when they need them. Initially, we assumed graduation was a binary state: either a site is large enough to host an election, justify a custom design, be ready for elevated privilege levels, and etc. But we've slowly come to the realization that not all sites develop the same way that Stack Overflow and other early entrants to the network have. In particular, some of my favorite sites don't get tons of questions as a result of their subject matter. So we are working on breaking up with graduation.
I know many of you are frustrated with the lack of progress, but the new theme has really made progress possible for sites that have been waiting far too long in beta. After a short respite at the start of the new year, we are finally ready to help more sites achieve graduation.
