Yes indeed, proper setup, proper maintenance, and definitely luck. Those belts, even the newest narrow versions, are extremely tough. But like anything else, they are subject to the occassional quality issues in manufacturing. Early failures that aren't the result of obvious damage can probably be blamed on such quality issues. But then there are things like improper belt tension, improper alignment, rough edges on a pulley, damage from foreign objects like small rocks, etc., that can definitely decrease the life of a belt. If my experiences are typical, the average dealer seldom sets belt tension properly, like when doing a supposed service or when changing tires, and it would be a good idea for owners to get a simple tension checking tool and learn how to check it themselves.
Of course, for those who just feel the need to hammer relentlessly on the equipment, there is always the option to convert to a chain drive. If I remember correctly, back in the day it was fairly common to see chains broken and thrown off, or sometimes hung up in the countershaft, but at least if you carried extra links and master links you could usually get back on the road. I've seen a lot fewer problems since the advent of belt drive, but if you want the best reliability I'd suggest a nice shaft drive.
Jerry
