If you read the little "journal" thingy (a published book that basically told his life story.. yeah, I'm a nerd), it was his cockiness that did him in. He could have finished off ObiWan but he gloated, and when Obiwan did the force leap over and around, he was too busy gloating to react in time, and was thus cut in half.
His arrogance did him in. Or so the official explanation goes. (FYI
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