One of the first things I mentioned to the creators is "Why not ARM" and the reason they gave is a pretty valid one, they wanted the platform to be easily accessible for beginners and switching to arm brings a quite a bit of extra work with it when just starting out, compared to an 8-bitter.
Even though stuff like mbed etc. are available their ecosystems are a lot less beginner friendly. This way the barrier to get started is put really low, but the gap between "bare-metal" and absolute beginner is a bit more manageable.