I think the reason is simple. Nokia really needs to build a momentum for N8 (and Symbian^3), which is supposed to be their top-selling mainstream smartphone for now. By announcing N9 they would totally ruin it.

The more clever way is to leak some blurry pictures, rumored specs and stuff like that, until the device is fully finished. That's the way how N9 stays in the headlines even though it's not even released.