I'm just going to go ahead and quote a response from a SE Community Manager (Ana), after I inquired about a similar question (see the full text in the link above):
This site is not showing the hockey stick growth we look for as a prerequisite to graduation. And it is entirely okay.
Now, a natural response here might be to push to artificially inflate the rate of posts, but please don't. It nearly always adversely affects quality, and quality is more important. You've got a good thing going here. Instead, know that the place you're at is absolutely fine. You're not set up to graduate anytime soon, but we're proud to have you as part of the Stack Exchange network.
This issue of low activity has been going on since the site came out of private beta, and if there was an easy answer, someone would have already changed it. Here are some recent discussions that relate to where we are as a site currently.
And finally, I have a graph here, charting the site activity over the past 282 weeks. Note that this past summer was quite the best in terms of activity. The 2014 jump was where I was exceptionally active, so not entirely natural. I am really liking how this summer looks! With that kind of growth we will move forward: