The way I see it the alpha is basically the blizzcon demo with some extra stuff enabled that they weren't able to access at Blizzcon. Hence why it seems to be only content creators who have been given access at this time (and some lucky folks). My hunch is there will be some pretty big patches and some pretty big invite waves soon that will bring the alpha up to where they are currently in development and a lot of the concern will fade away at that point.
I seem to recall something similar happened with Legion. The alpha was unusually long. I received my Legion Alpha invite 30th March. According to Google the Legion Alpha began in November. It then went into beta status. I think March will be a pretty busy month in terms of patches and invites.