Great history and story...thanks for taking the time to write that up!
What the history teacher said about it was that most cowboys could not afford to buy ammunition to practice to be good shots. When you are paid 50¢ to 75¢ a day, AND have a daily deduction for the mess wagon and another for board if you lived in the ranch house, you didn't have much left to spend on buying ammunition to shoot up target practicing. They'd rather spend their money on the rare trip into a town for drinking, women, or gambling. . . plus what ever other necessaries then might need. They were not very good shots. . . especially while drunk which is when most gun fights took place!