The merger between Publicis Groupe and Omnicom Group, announced in July 2013, was intended to create the world's largest advertising company. However, the merger fell through in May 2014 due to several issues:

1. Cultural and Leadership Clashes: One of the major challenges was the cultural clash between the two companies and their leadership. Publicis, a French company, and Omnicom, an American company, had different corporate cultures and leadership styles, which led to disagreements on governance and management structures.

2. Regulatory and Tax Concerns: The companies faced regulatory hurdles and tax complications that made the merger process cumbersome. The deal needed approvals from regulators in several countries, adding complexity and uncertainty to the merger.

3. Complexities in Structure: The planned merger involved a 50-50 ownership split, which made it difficult to resolve issues of control and where the combined company would be headquartered.

4. Client Conflicts: Both companies had several overlapping client relationships, which could have led to conflicts of interest and required clients to restructure their contracts or agency relationships.

5. Operational Integration Challenges: There were significant challenges regarding how to effectively integrate the operations and align the technology platforms of both companies, which proved to be more difficult than anticipated.

Due to these insurmountable differences and obstacles, both companies decided to maintain their independence and called off the merger. The decision was amicably reached, with both firms acknowledging that the deal was no longer in the best interests of their respective stakeholders.

For more detailed information on the reasons behind the failure of the merger, you can refer to this source.