Self-Regulated Strategy Development (SRSD) is a method designed to explicitly teach students self-regulation skills and help them implement them. While SRSD's beneficial effect on writing quality has been confirmed in a large body of literature, the questions of why, how, and for whom SRSD works remain unclear. To investigate the underlying cognitive and metacognitive mechanisms, and identify the factors that potentially moderate SRSD's effectiveness, we compared fourth- and fifth-graders who either received regular writing instruction or underwent an SRSD intervention in a quasi-experimental setting. Students' abilities to plan, write quality texts, and self-evaluate were measured. Factors that have been demonstrated to be related to writing abilities were assessed to study whether they might facilitate or hinder acquisition of strategies taught in the SRSD intervention. The results indicated that compared with students who received regular writing instruction, students who underwent the SRSD intervention produced higher-quality texts and evaluated their texts' quality more accurately, despite persistent overestimation. Mediation analyses indicated that progress in producing high-quality texts was explained by improvements in students' planning skills, enabling them to write stories that were better-structured and contained more ideas. However, the results also indicated that some students under the SRSD condition struggled to implement the planning strategies they were taught, particularly students with poor working memory. Recommendations for optimizing the SRSD intervention's effectiveness in developing all students' writing skills are discussed.