What might explain the disappointment of the CTO due to several increments released without user acceptance testing?

The disappointment of the CTO can be attributed to the situation where user acceptance testing (UAT) was not included in the Definition of Done. The Definition of Done is crucial for ensuring that all necessary steps, including UAT, are completed before an increment is considered finished and ready for release to users. If UAT is omitted, it signifies that increments may not have been adequately validated from the users' perspective, leading to potential issues with product quality and user satisfaction.

Including UAT in the Definition of Done ensures that the development team conducts necessary testing with actual users or representatives to gather feedback and ensure the product meets their needs before final delivery. This oversight could lead to the release of increments that do not align with user expectations, ultimately frustrating the CTO and other stakeholders who anticipate a product that has been thoroughly vetted for user acceptance.

Focusing on including all critical processes in the Definition of Done emphasizes the importance of quality and user involvement throughout the Scrum process, ensuring that the development aligns with user needs and reduces the disappointment that can arise from untested increments.