It's been over a year since we voted to have a Pyro update (before there would be a Heavy update).
Since that point we haven't had a real Halloween update (just a reactivation of previous updates with some new community items). The Christmas update was more of the same, but since Christmas updates have primarily been about cosmetics in the past I guess that's not as big of letdown. This was also the first year without a major summer update.
Theoretically the reason for all of these missed updates is that they are working really hard on the Pyro update, and maybe the later Heavy update.
I mean, why would they make us vote on which update would come first if they didn't plan on doing any updates at all? Surely Valve wouldn't commit themselves to a project and then just drop it entirely when they got bored/distracted/found it harder than expected!
So sarcasm aside, I'm not sure if people are really expecting an update at this point, since we're all aware of how much Valve has let us down. But Valve certainly promised an update (in fact, they promised two updates).