iPhones and iPads already have the ability to have user-made code run on them in various forms (so testing can be done) the internal drive is accessible in disk or DFU mode, they have a much wider audence of people who know about the system in general already (due to the existnce of similar platforms and older OS versions for a few years), and they have very firm benefits to hacking them as far as usability (that is, not for playing games).
3DS? Does it's best to not let the user run anything, no direct access to the NAND, the audience is one that's dealing with a new unit and OS, and outside of running some code portably (for which there's other way easier methods, like a smartphone), there's not much practical use in hacking them.