Background: Software Process Improvement (SPI) is intended to improve software engineering, but can only be effective if used. To improve SPI's uptake, we should understand why organizations adopt SPI. CMM-based SPI approaches are widely known and studied. Objective: We investigated why organizations adopt CMM-based SPI approaches, and how these motivations relate to organizations' size. Method: We performed a systematic review, examining reasons reported in more than forty primary studies. Results: Reasons usually related to product quality and project performance, and less commonly, to process. Organizations reported customer reasons infrequently and employee reasons very rarely. We could not show that reasons related to size. Conclusion: Despite its origins in helping to address customer-related issues for the USAF, CMM-based SPI has mostly been adopted to help organizations improve project performance and product quality issues. This reinforces a view that the goal of SP...