Mostly because "alpha" means "alpha" not "release candidate".

Ask yourself: do I want to associate my brand with the product in it's current state? If it is alpha quality but you still want feedback, think damage limitation. It's no accident that programs get code names: you associate all of your alpha woes with a brand you can throw away.

It's a bit like throwing a party. If we open the doors and we're the only ones home (as awesome as we are), people who stop by are less likely to stay and get involved than if there's a crowd of people already partying that they can join.

So we're keeping the alpha closed at first, so we can fix things and grow our userbase to a decent size without embarrassing ourselves. We do want to open up as soon as possible after that.