I've asked myself the same question many times...
Maybe good genes and a healthy diet? Just kidding of course.
Personally, I think is to do with God wanting to show us that HE is the One in charge. He can give life, prolongs life an take it away when he chooses to.
I believe that it was also a sign of Prophethood. One could hardly denie such a miracle and it would make the message they had easier to be believed.
It's all I got.