This seems likely. Early iPhones and iPod Touches were much more attractive targets for jailbreaking, given their more generalized nature, better hardware, and more capable native OS — the range of possibilities was far more great.
It's also not practical to upgrade the storage in flash-based iPods, which reduces the utility of something like Rockbox. The stock OS is perfectly serviceable for the size of library one would be carrying around on a Nano.
