I'm guessing the lack of sufficient time since principle photography wrapped for the FX house(s) to actually generate enough footage that will actually be in the movie, featuring the finalized Godzilla design that Edwards and his team chose, is the only reason we haven't seen anything yet. They have to manufacture these FX scenes before we can see 'em!
I'm also wondering if LP & WB aren't more than just a little leery of showing
any FX scenes before they're at least 99% finished. I recall the internet backlash when the original IRONMAN promo spot was shown during the Superbowl. Lots of griping about how CGI looked to fake, etc., etc. If memory serves, Favreau & the ILM team took that criticism to heart and actually tweaked the scene (IronMan blows up the tank) to make it better.
LP & Warners may be trying to avoid any and all bad press before the film gets released, which is good crowd control - up to a point. That said, hiding Godzilla himself from everybody is a bad move because everyone interested in the film will just assume he looks too bad-CGI/cartoony or that the final design itself is too far removed ala GINO, or whatever their net rants will say. *SIGH* I'm just counting the days until we get some stills or legit footage of the monsters. *twiddles thumbs*