I think the issue with high learning curve tools is not that the learning curve is high, it's that the benefit seems (rightly) dubious at the beginning.
You can find videos of people whizzing along with emacs or vim, and it's not difficult to see the productivity gain. So it'd be cool if you had a video of what it looks like to program with this thing.
That said, I'm not trying to convert people just yet. It still needs polishing and support for a few more languages (right now it only does Lua, Python and basic Lisp, which is not very exciting).
1) Premature open-sourcing complicates commercial projects, and a man gotta eat.
2) Because it would be a first, any flaws in the implementation will be assigned to the concept of structured editing itself, hurting the development of alternative implementations. Even if my vision of it fails, I want other people to have a shot without fighting incorrectly formed opinions.
In the end, it's the ideas and not the source that are important. It's like developing for VR now, or mobile a few years ago. The space of possible interfaces is so big that most of the effort goes into combing it.