I had the same problem and it frustrated me for months. Quick look used to work for all my comic book files. Then suddenly it worked for cbz but not for cbr files. I think it happened with the Mavericks update but it may have been earlier.
Anyway, tonight I finally fixed the problem. Here's how. I hope these steps help others who have been similiarly frustrated.
1. delete your archive app (in my case it was unRarX);
2. delete Simple Comic;
3. restart;
4. install archive app (e.g. unRarX);
5. in Finder, right click a cbr file and choose 'Get Info'; via 'open with' set your archive app (e.g unRarX) as the default app for rar files; (NB - this is the crucial step);
6. restart;
7. install Simple Comic;
8. open Simple Comic;
9. restart;
10. open Finder; look at a folder with cbr files, in cover flow mode; you should be happy with the result 🙂
P.S. the key appears to be the fact that without any 3rd party archive app and without Simple Comic, the .cbz format has an app which will open it - Mac OSX's own 'Archive utility'; .cbr on the other hand, has none; if you put the two file formats on an equal footing before installing Simple Comic, i.e give .cbr an app that will open it, Simple Comic applies its Quick Look plug-in to both formats in a way that Mac OSX (Mavericks) understands;