Powerful business tycoons like Carnegie and Rockefeller were called both "Captains of Industry" and "Robber Barons" because

They used questionable methods for reaching their wealth level.

Robber baron was applied to powerful nineteenth-century industrialists who were viewed as having used questionable practices to amass their wealth. On the other hand, "captains of industry" were business leaders whose means of amassing a personal fortune contributed positively to the country in some way.