"We want Arc to be good for writing programs, and one way to ensure that is to start writing programs while the language is still malleable. In the process we've learned a few lessons.

Then doesn't it stand to reason that putting up a pre-alpha tarball or giving read-only access to a source repo would allow you to learn more lessons about your fledgling language.

I can see two problems that Graham might be trying to avoid:

1) Arc is bound to aggravate some Lispers with whatever choices are made in the final syntax. It's an impossible group to please. Whatever arcane decisions he reverses will be inevitably disliked by some subset of the people paying attention. Perhaps he hopes to avoid this by not showing off the language too early while it's still "malleable" enough that the unwashed masses think they have a shot of getting their favorite cruft shoved back into the language via noisy publicity campaigns. Think ugly language wars on Usenet meets Technorati.

2) Perhaps he thinks that him and his friends are the ones best suited to determine what should be in Arc and just isn't that interested in what the rest of us plebes think. That would tell us a lot about what kind of governance Arc will have: authoritarian. Think benevolent dictator for life without the benevolence.

Of course, neither of these could be the reason. It's hard to divine from the tea leaves and goat entrails left in a few web posts over the course of six (!!) years. That said, I'm dying to give it a spin.