Ask HN: If Django's admin is so awesome, why hasn't it been ported to Rails? 17 years ago
I think that it has a lot to do with what each system was designed for. Both are great and can accomplish nearly the same things, the difference is that django was built for newspaper websites originally, so the focus on a standard admin CMS makes a whole lot of sense, especially when you are going to be using the same codebase on 5,10,20, etc... sites.
Rails was built around making client websites and in house web apps for 37 signals. Thus, perhaps on an app like Basecamp or many client sites, a standard admin panel wouldn't make a lot of sense. Something more custom would need to be built for those.
I find that most devs tend to "scratch their own itch", so that might explain the difference.