Well, your question is significant in that the name of the child is very important and, in fact, becomes a self-fulfilling prophesy.  The baby, however, is not JUST named Luck, he's named Thomas Luck (and is affectionately nicknamed "the Luck" and also "Tommy Luck").

The significance and the "why" of the name have to do with the setting.  "The Luck of Roaring Camp" is set in the late 1800s, at the height of the gold rush in California.  Roaring Camp is the name of the mining camp that the men live in.  The baby is birthed by a Cherokee prostitute, Cherokee Sal, who dies soon after the birth.  The men decide as a unit to care for the baby as their own.

Now to the heart of your question: the men name the baby Luck because the men believe the baby has brought the quality of luck to Roaring Camp.

The real luck of Luck manifests itself as these rough men take better care of themselves and their surroundings.  

They’ve got vines and flowers round their houses, and they wash themselves twice a day. But they’re mighty rough on strangers, and they worship an Ingin baby.

They clean themselves.  They clean their cabins.  Their shouts have been replaced by whispers.  They huddle around the infant instead of huddling in seedy places.  They no longer participate in as much gambling and fighting.  Luck truly changes the men.