Randoset, it is difficult to know what went wrong if anything. First, was the brisket select, choice or prime. I firmly believe a better cut makes for a more tender end product. Also, you didn't mention what the internal temperature was when you pulled it. There is a big difference between say 165 and 190. I noticed that you cooked yours at 200, so maybe it simply wasn't in long enough. I did a 10.69 pounder for Thanksgiving at 225 and it took just over 16 hours. Finally, just keep trying. It took me quite a while to get to the point where I consistently have success. Rest assured, this forum is one great resource.
Best wishes,