The Pi Zero seems to be a more advanced and complete solution than the compute module (it has many feature the second does, except the number of IO). But it is way cheaper, even when adding the cost of a memory card.
Is there a reason for that?
The Raspberry Pi Zero is produced and distributed by the Raspberry Pi Foundation directly. The other Raspberry Pi boards have commercial partners involved with their sales. These partners improve the availability of the other boards, at the cost of incorporating a profit margin for everyone in the supply chain.
The Raspberry Pi Zero is still deliberately not available in multiple quantities to individual buyers. It is meant to provide the cheapest computing platform for those who need it, rather than those who would like to integrate it into a commercial product. The Compute Module is meant for larger volume integration.
Requests for volume supply of Raspberry Pi Zero boards are promptly shut down on the Raspberry Pi Forum run by the Foundation. They don't appear to even want to discuss the decision.