To support organizations in their greening efforts the normative and instrumental 'how to' perspective is indispensable. However, the descriptive 'why not' perspective adds valuable insights to factors that may hamper, decelerate or even block green purchasing. Our study follows Eisenhardt's design for case study research and comprises three stages: in the first stage, we analysed barriers to green procurement in six municipalities in five European countries; in the second stage, we combined those findings with the literature on multilevel research, present a barrier framework based on a multilevel categorization and develop hypotheses by analysing the organizational and the individual level in one of the biggest procurement agencies in Europe. In the third stage, we test the derived hypotheses and draw conclusions. In order to be able to deeply analyse one specific decision process and its inherent barriers we focused on the procurement process due to its gatekeeper position in organizations. We contribute to understanding the greening of organizations via the 'why not' perspective by answering the question 'Why do organizations not implement green purchasing?' As organizations are multilevel systems, this paper illustrates how the differentiation of the multiple levels of analysis and the detailed analysis of one specific task can contribute to understand greening organizations and moreover to continuously improve greening in organizations. Our study is designed as a multilevel analysis examining the relationships between the individual, group, organizational level, and the external environment that lead to barriers for green purchasing. The multilevel framework can be used for analysing barriers in green purchasing. Complementing the 'how to' perspective with taking the 'why not perspective' we look on the challenges managers, who foster sustainability, are confronted with.