I find that people tend to look for a singular cause; a trail of evidence that leads to only one correct conclusion. It's an attitude exploited by crime dramas, TV shows, literature, etc.

Reality, of course, is far more complex, and I think Rosling is trying to play to those biases in human nature in order to influence people. Pointing to one inherent "truth" in the data is far more effective in convincing people than "it's complicated".