Mostly I think it's because anything outside of the WWDC tends to be a focussed events. When they have the iPad event - it's just about the iPad. Pretty much every iPhone event has just been about the iPhone. The iPods are pretty much the same. Right now we have no evidence, not even from well placed pundits like Jim Darlimple or John Gruber saying that this event is anything but iOS 6 and the new iPhone.
If new iMacs are coming its not going to be in a keynote - I think they will be a silent upgrade on the website and that's it. Apple hasn't been giving a whole lot of love to their desktop line and I don't see that changing much.
Wasn't the 2009 27" iMac release (not really a spec bump and not really a redesign) a quite release?
http://www.redmondpie.com/apple-announces-new-27-imac-mac-mini-macbook-and-magic-mouse-idx359/
I really can't remember.