CBP has refrained from any clear statements on this point, but my impression is that there are three issues that may be contributing to keeping the U.S. from participating in pre-clearance processing:

1) Security concerns
Right now the U.S. has total control over the application process for U.S. visas, including required interviews. However, if they start granting pre-clearance approval (and therefore visa-free access) to foreign ABTC cardholders, then they will have to rely on the judgment of the card issuing authorities in the cardholders' home economies. I think the U.S. is a little uneasy about trusting other governments to properly screen ABTC applicants, especially since no interviews are required.

On the other hand, the 19 full member APEC countries are comfortable with this process, including immigration/security-sensitive economies such as Australia, Japan, and China. Hopefully now that CBP is finally issuing ABTC cards it will become more familiar with other economies' screening practices and get over its uneasiness.

2) Budget concerns
CBP's current plan to issue ABTC cards does not require any major investments on their part (other than printing cards) because all applicants are already screened through existing programs like Trusted Traveler. However, if CBP had to submit and review pre-clearance applications to/from other economies, they would need to make substantial new IT and staff investments to support these activities. It sounds like budget constraints may be limiting their options in this regard.

3) Privacy concerns
It sounds like CBP has some concern over how the data of U.S. cardholders will be protected if it is submitted to other APEC economies for pre-clearance review. Although again, 19 other economies don't seem to have a problem with this. This may be another case where CBP just needs to become more familiar with practices in other economies.