It's my understanding that Tesla is still in the early stages of the ramp up, so they are still doing a lot of manual building (vs. automated). Also, since they haven't released many details about the vehicle, a lot of people have been waiting to make the commitment. Only the truly faithful have confirmed (thank you).
I suspect that all these configuration emails have gone out were needed to find enough faithful customers in order to get enough vehicles to build. Once the details come out, I have a feeling that a lot of people will confirm and then we won't see any new Emails for quite some time while Tesla works through their new backlog. Of course, it will all depend on how fast they can ramp up.