Collaborative contracting models are increasingly being adopted for complex construction projects but achieving consistent improvement and broader alignment of such practices remains challenging. Based on a longitudinal study of collaborative contracting in Swedish infrastructure construction, this article argues that legitimacy concerns among major public sector clients contribute to this difficulty. Top management often responds to external criticism by quickly adopting new models, thus delegitimizing earlier practices and disrupting continuity at the operational level. Consequently, learning is cut short, and performance gains are unrealized—leading to repeated cycles of short-lived collaborative initiatives without sustained improvement.