Same reason it's better for humans to have children before their 40s. Bettas are in their prime before the 1st year. This is when they have the most energy, are best able to cope with the physical stress of spawning, and make the best parents. Sex drive usually diminishes with age as well, so older fish are less likely to spawn in the first place. I'm not sure if this applies to fish or not, but the older a human female is when she becomes pregnant, the more likely the chances of birth defects in the child. For instance, Down syndrome occurs in only one out of every 1,600 children born to a mother in her early 20s, but the odds increase to one out of every 100 if the mother is in her 40s!