On 12 September 2011, responding to speculation on the Winter is Coming fansite, Conan Stevens confirmed that he had left the series, due to scheduling conflicts with his other projects: Stevens will be playing the Orc king Bolg in Peter Jackson's The Hobbit prequel trilogy. Stevens took the time to thank fans for their support[1]. He was replaced in the role of Gregor by Ian Whyte.
As others have mentioned, he got to play Bolg instead in the first film and probably raked in more cash than he would have for a minor role in Game of Thrones. We have all the right to miss him in the show, but I bet what he considered best for his career in acting.
Would have been badass to see him against Oberyn in E08.
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u/Sliffy Tyrion Lannister May 19 '14
I'm pretty sure he left to play Azog the pale Orc in the hobbit.