Real-world gas mileage of the manual Cruze Eco is very good, better than EPA. The automatic Cruze Eco does not do anywhere near as good as the manual, although it is better than the regular automatic Cruze LT/LTZ.
The diesel is meant to deliver Cruze 2LT/LTZ levels of refinement with Cruze Eco manual fuel mileage. The diesel has options that the Eco with either transmission cannot get, such as leather and a enhanced technology package. It's also quieter thanks to more sound deadening. The diesel also gets larger brakes front and rear, along with cushier struts. The comparison between the diesel and the Eco is apples to oranges since they are very different cars underneath the skin. Fraternal twins is an apt comparison.
Also, not everybody wants a high-MPG car with a manual transmission. While I and other posters in this thread do like manuals, other folks might not. And, we're in the decided minority, albeit a very vocal one.