Plenty of reasons, but I suggest not dwelling on why you failed. Focus on your strategy to pass the exam. Use Andy Crowe's book AND the free Head First exam questions. It's the route that I took - I have not even seen the PMBOK!
During the exam, you should feel confident that you'll pass. And once again, stop stressing the fact that you FAILED. It's not an easy exam. There are plenty of people who required taking a second or even a third attempt. Your PM experience will help but focus on the PMI way and you'll be golden.
These from a guy who passed on his first take. Key concept: Understand and don't just memorize.