The story takes place in the mythical town of Calivada where Lightnin' Bill Jones, or more correctly his wife, operates a rather seedy hotel that straddles the California-Nevada state line convenient for those looking for a quick Nevada divorce. He is nicknamed Lightnin’ because, as the local postmaster put it, “We call him Lightnin’ because he ain’t.”
Lightnin’ Bill, a Civil War veteran known to brag that he advised General Ulysses S. Grant, also claims to be a jack of all trades, having been at one time or another, a judge, inventor, detective and bee keeper. Of the latter profession he spins the tale that he once drove a swarm across the prairie in the midst of winter without the loss of a single bee. When pressed Lightnin’ Bill concedes that during the drive he may have been stung once or twice.
Lightnin’ Bill likes to spend his days and nights carousing with cronies rather than being at home with his wife and adopted daughter. When he refuses to go along with the sale of the hotel to group of out-of-town businessmen, his wife becomes furious and files for divorce. In court Lightnin’ Bill, with the help of young John Marvin, is able to prove that the buyers are unscrupulous scoundrels and wins back the love of his wife.

Question: Who said, "We call him Lightnin' because he ain't" ?
Answer: A local postmaster
Alternative answer: The local postmaster