Halo 2 LE disc they talked about how when they came back from the E3 demo that they really didn't have a real plan. They spent a majority of their time preparing that demo (along with some of the graphical features in it) only to come back and figure out that they needed to start over on some of their story and game feature set (dropping or not using them; not redoing the entire fucking engine who ever said that).

That's all the E3 demo actually was...an engine demo.

Also, after Halo 2 the rings were put on "standby" due to mid-sequence shutdown...hence the need to go to the Ark.