According to Grandma Dowdel, the true reason Shotgun Cheatham died penniless was because he was "an old reprobate" who failed to make something of himself while he was alive. He "lived poor and died broke." In other words, Shotgun Cheatham was a ne'er-do-well.
Grandma Dowdel tells how Shotgun Cheatham got his name, and her story exemplifies Shotgun's incompetent nature. Accordingly, Shotgun had gone out to shoot quail with a group of older boys when he was ten. Even at that age, Grandma maintains that Shotgun was a bad shot: "He couldn't hit a barn wall from the inside, and he had a stye in one eye." Shotgun was so happy to be out shooting with the older boys that he shot off a round recklessly. As a result of his actions, he unintentionally killed a cow.
Upon seeing the results of Shotgun's actions, the older boys took the gun out of his hands, for fear that Shotgun would end up hurting someone. This is how Shotgun got his nickname; despite this, though, Grandma maintains that any girl in her time could have out-shot Shotgun easily, so incompetent was he with a rifle.
So, Shotgun Cheatham died penniless because he never made much of himself when he lived: "He lived poor and died broke."