From the blog:
we’ve not been able to build them, because to do so would mean that we have to cannibalise Model B parts – and that would mean that people who are experiencing the backlog would have to wait even longer
Of course, you would not want to let people wait a few weeks for a Model B. People who have been waiting for months for a Model A on the other hand... I said it before elsewhere, and I repeat it here: just raise the price of Model A to bring its profit margin on par with Model B, and let the market decide what it wants! The reasoning that $25 was crucial to reach education is not even used to justify Model A anymore. Now it's for robotics, automation and media centers.