What's keeping Shotwell from replacing EOG?

I have noticed that Shotwell has a pretty capable image viewer but EOG is still used for opening image files, which means Ubuntu's default install has two apps for the same purpose.

I wonder why Shotwell isn't set as the default image viewer and EOG is removed. Are there certain features that EOG has and Shotwell lacks, which are keeping it from being set as default?

Note: I work for the Yorba Foundation and am the lead developer of Shotwell. The following are my own opinions and observations.

When Ubuntu first approached us about making Shotwell the default photo manager, we suggested to them that Shotwell not replace EoG as the individual photo viewer. At the time Shotwell's photo viewer lacked many significant features that EoG offered, including pan-and-zoom and a thumbnail strip view of all images in the current directory. EoG's metadata viewer (Image -> Properties) is also more thorough than Shotwell's.

That said, we're constantly looking to improve Shotwell and all these items I've mentioned are slated for inclusion at some point. (Pan-and-zoom has been added since Ubuntu's decision.) If Shotwell had a thumbnail strip view (or even a sidebar listing all files in a directory) I would feel comfortable recommending it as a default photo viewer – but that's a decision for the distro, not me.

I guess that Shotwell is more of a complete photo management application than a simple image viewer like EOG.
That's why the official statement from ubuntu talks about it as a replacement to F-Spot.

This does not exactly address your question, but you can change the default viewer.

If you right click on the image, select "Open With", then "Other Application...", leaving 'Remember this application for "JPEG Image" files' checked, find 'Shotwell Photo Viewer' and click "Open" it will set Shotwell Photo Viewer as the default viewer.

I find Shotwell Photo Viewer faster to open than EOG--which can be removed using your favorite package management tool.

As far as the difference in features, I can not see a significant difference.