I think the problem is that it's so cheap it undercuts all the other more expensive models that have higher margins. When you look at adafruit's website for example, they still have plenty of zeros packaged in kits that sell for 30+ dollars but bare zeros have been out of stock for months.
I think the problem is that it's so cheap that the foundation don't want to make many of them. It doesn't actually seem to sell all that fast compared to its bigger brothers from what I can tell, but there's so few of them produced that they sell out rapidly every time they're in stock.