Scheduling conflicts most likely. It's a shame as he was perfect for the role.
Considering how many books there are and how long it will take for each to be adapted onto TV, I'm wondering if any of the major characters will have to be replaced in the future.
The actor himself made a statement on his website saying that he isn't sure why he was replaced either, but that he was thankful for the chance to have the role.
wait what? is it on any of hbo's sites LEGITIMATELY? i should have access to all of them
doesnt look like it, guess maybe they uploaded it to a server or something and someone pulled it off. that or maybe it was sent to the media and they leaked it (since didn't they review the first few episodes already)?
Edited:
did they recast gendry too? god damn stop recasting people
I think with Gendry the actor has gotten older and as a result looks somewhat different, I'm not sure myself though.