The establishment of a responsibility matrix is crucial in a hybrid cloud approach because it clearly outlines the security responsibilities of the cloud service provider and the client. It ensures that both parties are aware of their specific obligations, which in turn minimizes gaps in security coverage. Understanding who is responsible for each aspect of security helps to prevent overlaps or omissions that can lead to vulnerabilities.

In regards to creating a common set of security policies and ensuring regulatory compliance, while these are important considerations, they come after defining the responsibility matrix, which lays the groundwork for security responsibilities and roles.