I'd think a major reason is that a big part of the players are not using Kaya as their main go server yet.
Like me, they're checking Kaya out, but not (yet) playing many games there, falling back to their main server while waiting for the client to finish the beta stage.

I think given how many accounts have been made available, and the fact that they're no longer going immediately, I'd be worried. It may not be "public", but it's not so hard to get an account anymore.

I have an account at Kaya and log on occasionally (mostly to see what features they have added while I was gone), but I almost never use Kaya to play games. That is because Kaya still hasn't implemented all the features you want for frequent game play. I don't think that's a problem yet. They are still working on the software.

The way I see it, Kaya is sharing a 50% ready product with the community in order to improve its development process. Therefore, I would not worry about the fact that it doesn't have many users yet.

Of course, the standard by which I evaluate Kaya will change a lot when they release the gold version of their product. Right now, the sausage is only half-made and not yet cooked. So when I evaluate Kaya, I don't evaluate it according to the same standards I evaluate KGS or Tygem. It seems premature to worry about how the hot dog will taste at this point. We normally don't see how the sausage is made, so this is kind of a new experience for most of us. We can use the other servers for gaming while we wait for the Kaya hot dog with mustard and relish. If no one is eating the Kaya hot dog after it's cooked, then we can all start wondering what went wrong.