Mermaid pose is easier to get into for most yogis because the shoulder of the binding arm needs mostly horizontal abduction and some extension (pulling your arm back) which are easier motions for most. This involves opening up the anterior deltoids and pectoralis major muscles. The top arm does a similar action as in the traditional variation, but it needs much less external rotation so less stretch of those tight latissimus dorsi, teres major and pectoral muscles. Also, the back bend in the upper spine doesn't need to be as deep in Mermaid.
Hope that helps!!!
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Anatomy images from Thieme Atlas of Anatomy: General Anatomy and Musculoskeletal System.