Bluefeet's answer is the canonical answer as to what happened today on SO. My answer is just an attempt to shed light on past practice and procedure.
There's a long history of this. If I have all the math and timing right, on SO alone,
Basically, as I understand it, a new election is called when there are spots to fill and it has been at least 6 months since the last election. Otherwise, open positions are filled as needed with the runners-up from the most recent election, if those individuals still wish to serve. Elections take a lot of community time and energy, so we generally want to limit them to 1 or at most 2 per year. SO has already had 2 in the last 12 months. As bluefeet said, though, the flag queue has been pretty full, and we need help sooner rather than later.
No doubt, Stack Overflow staff can shed additional light on this.