We don't know
Rather than provide details on Durin I (aside from inclusion in the creation of the dwarves), Tolkien seems to have included him as more as a legend.
Durin is the name that the Dwarves used for the eldest of the Seven Fathers of their race, and the ancestor of all the kings of the Long-beards. He slept alone, until in the deeps of time and the awakening of that people he came to Azanulbizar, and in the caves above Kheled-zâram in the east of the Misty Mountains he made his dwelling, where afterwards were the Mines of Moria renowned in song.
There he lived so long that he was known far and wide as Durin the Deathless. Yet in the end he died before the Elder Days had passed, and his tomb was in Khazad-dûm; but his line never failed, and five times an heir was born in his House so like to his Forefather that he received the name of Durin.
Appendix A - III: Durin's Folk
We have no information to suggest a reason for his longevity. We simply know that he was "born" before 1250 Y.T. and died before the end of the First Age (which was roughly 19,000 adjusted years later).