I would guess that they were one at a time because demand has been so high. By the time I was finally able to order a zero I lost interest in it and started using other boards. The question is if you want to get it into as many hands as possible or to people that are going to use a lot of them. But also people bulk order and sell at a markup. Stuff like this[1] still isn't uncommon ($23 for a pi zero on Amazon). At that markup I'd rather just buy a pi, arduino, or whatever. At $5 if it is a zero vs a msp430 I'm getting the zero for ease of use.

[1] https://smile.amazon.com/Raspberry-Zero-v1-3-Development-Boa...