strains: Some strains are more fragile and will live less long. Primarily because the colors are more recessive, so the strains are a bit more inbred, lowering their resistance to diseases. So a disease comes, goes and takes the betta with it. 

spawning: Breeders will usually live less long, because breeding bettas is stressful for them and takes its toll. In short, die hard single males live longer LOL.

care: the better you care for your betta, the longer it will live. Clean water is a must. Keeping a healthy environment for your fish insures that his stress levels are low and immune system levels up. This will prevent diseases to take a hold of him. A betta that gets sick might recover, but usually will not live a long life thereafter, for it appears to always remain weakened.

space: the more space and exercise your betta gets, the longer its life expectancy. An experiment was conducted to this regard, where a betta was kept alone in a very large tank (I believe it was a 50 gal) and it was provided exercise every day by being gently chased around a bit with a net. This specific betta lived to be 5 years old. One might however argue that chasing your fish around would stress it out and may cause it to get sick, but for that one fish it seemed to work. I would not recommend it though.

diet: A good varied diet, supplemented with vitamins, would give the betta all the nutrients it needs to better deal with the aging process.

last but not least, GOD ;). I guess some of us are meant to live longer than others. So some of it may be destiny I guess.