A clash of personalities was the key reason behind the collapse of the Publicis-Omnicom blockbuster merger, the head of the world's largest advertising firm WPP told AFP on Friday. WPP chief executive Martin Sorrell said the "primary problem" behind the deal's failure was "the clash of two strong-willed CEOs who, prior to the announcement, announced they could work together -- and we all know that mergers of equals don't work." Sorrell spoke after French group Publicis and Omnicom of the United States announced late Thursday they were scrapping merger plans that would have created a company bigger than London-based WPP. While the companies have complained of tax and regulatory snags, the Wall Street Journal reported last month the project had become a "battle of titans" between chief executives Maurice Levy of Publicis and John Wren of Omnicom Group. Levy said Friday the failure of the merger doesn't leave Publicis worse off as it "was never considered a necessity" but "a good opportunity". However, Sorrell added on Friday: "I think they're in a difficult position because a few weeks ago they would (have tried) to say that together is beautiful and now they're saying being apart is beautiful. "It's a bit difficult to justify why they took ten months to find out that they couldn't get together and spend a couple of hundred millions dollars finding out." He also noted that any tax issues or structural issues over the proposed deal "could have been fixed". Questioned about whether he was surprised about the collapse, Sorrell replied: "We are not surprised. We are surprised by the speed with which it fell apart ... but it looks like both sides have decided that the deal lacked strategic purpose and structural clarity, and therefore they have decided to can it, as they would say in America. "I think Maurice (Levy) said it was 'an inability to agree on anything'... so there was a fundamental disagreement. "This is the sort of thing that you see on 'Mad Men'," he added in reference to the hit cult US TV series about a 1960s New York advertising agency. Sorrell said WPP would continue its strategy of making small medium-sized acquisitions, and that while its French and US rivals were busy arguing it had taken a lot of business away from them. "While Publicis and Omnicom were arguing, they've left the field open to us."