Obviously there are many reasons why one does not finish a triathlon. Injuries, accidents and weather conditions are mostly events you can't control yourself. There are actually a few reasons you will not finish your first triathlon and there will be only one to blame....you...
1. Unable To Change A Flat Tube
Yes, there are people that are not capable of changing their own tube. Make sure you change one tube the week before the race. Time it and check where you can improve. Changing a tube is a matter of taking it step by step. If you miss one step it will take you so much more time. And remember, there are guy's that had 5 (!) flat tubes during a race, read all about it here http://nikimathias.com/body/how-exactly-did-i-get-five-flat-tires/
2. Poor Nutrition
You have trained 6 months for your first race, you are fit like you have never been before. And now, 15K in your half marathon you are done. Exhausted. You try to continue but because of the fact you did not drink enough you are about to collapse. Game over. This is just an example of how important it is to eat and drink by the clock. The rule of thumb is to make sure you take an average of 250 calories per hour. Experiment with different kinds of nutrition but don't experiment during your race, stick to your routine.
3. Swim Panic Attacks
The triathlon start can be very hectic with hundreds of swimmers entering the water within seconds. Remember, no one cares about anyone else, it is all about you now. People will swim over you, drag you underwater and slow you down in front of you. Again, this could have been practiced before during open water swims with friends. To prevent getting into the 'washing machine' it is wise to either start in front, on the side or in the back. Obviously the front swimmers are very confident they will be FAST. Personally I like to start in the front on the side.