The screen used to be a module that was encapsulated inside a metal shell and covered with a glass layer. This meant the module itself had to have at least some degree of self-containment as an independent module, with some form of backing (however thin/flimsy) and a layer of glass to hold the "LC" (liquid crystal) in.

Now the screen IS the display module. The metal shell is the actual backing of the LCD module, and the glass is the actual LC-retaining surface.

As for not touting improvements in depth & weight: there's only so much trumpeting of awesomeness that the audience can take before they glaze over. While not the only selling point, the screen is a major factor their advertising group is focusing on. Even without prominent declaration, you & I & others "get it" that it's lighter & thinner. It's not that Apple is neglecting a miracle of engineering, it's that flouting too many miracles at once numbs the audience and degrades the experience ... which methinks is why we did not see new iMacs: one piece of overwhelming awesomeness is enough for the month.